# Full text of "Elements of differential and integral calculus"

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CORNELL UNIVERSITY LIBRARY 3 1924 031 219 326 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 924031 21 9326 NEWCOMB'S Mathematical Course. I. SCHOOL COURSE. Algebra for Schools, $1.20 Key to Algebra for Schools, .... 1.20 Plane Geometry and Trigonometry, -with Tables, 1.40 The Essentials of Trigonometry, .... 1.2S II. COLLEGE COURSE. Algebra for Colleges $1.60 Key to Algebra for Colleges, .... 1.60 Elements of Geometry, 1.60 Plane and Spherical Trigonometry, with Tables, 2.00 Trigonometry (separate) 1.50 Tables (separate), 1.40 Elements of Analytic Geometry, . . . 1.60 Calculus (in preparation) Astronomy (Nenrcomb and Holden,) . . . 2.60 The same, Briefer Course, .... 1.40 HENRY HOLT & CO.. Publishers. New York. NEWCOMB'S MATHEMATICAL COURSE ELEMENTS DIFFERENTIAL AND INTEGRAL QALCULUS ' - n 'I' Ok:,/ BY ' '' t. h h Y siko^ :n;ewoomb Professor of MathemaUcs in the Johns Hopkins University NEW YORK HENRY HOLT AND COMPANY 1887 CoPYiaoHT, 1887, BY HENRY HOLT & CO. Ul PREFACE. The present work is intended to contain about as much of the Calcuhis as an undergraduate student, either in Arts or Science, can be expected to master during his regular course. He may find more exercises than he has time to work out; in this case it is suggested that he only work enough to show that he understands the principles they are designed to elucidate. The most difficult question which arises in treating the subject is how the first principles should be presented to the mind of the beginner. The author has deemed it best to be- gin by laying down the logical basis on which the whole superstructure must ultimately rest. It is now well under- stood that the method of limits forms the only rigorous basis for the infinitesimal calculus, and that infinitesimals can be used with logical rigor only when based on this method, that is, when considered as quantities approaching zero as their limit. When thus defined, no logical difficulty arises in their use; they fiow naturally from the conception of limits, and they are therefore introduced at an early stage in the present work. The fundamental principles on which the use of infinitesi- mals is based are laid down in the second chapter. But it is not to be expected that a beginner will fully grasp these prin- ciples until he has become familiar with the mechanical pro- cess of differentiation, and with the application of the calcu- iv PREFACM. Ins to special problems. It may therefore be found best to begin with a single careful reading of the chapter, and after- ward to use it for reference as the student finds occasion to apply the principles laid down in it. The author is indebted to several friends for advice and assistance in the final revision of the work. Professor John E. Clark of the Sheffield Scientific School and Dr. Fabian Franklin of the Johns Hopkins University supplied sugges- tions and criticisms which proved very helpful in putting the first three chapters into shape. Miss E. P. Brown of Wash- ington has read all the proofs, solving most of the problems as she went along in order to test their suitability. CONTENTS. PAET I. TEE DIFFEBENTIAL OALOULUS. PAGE Chaptek I. Op "Vabiablbs and Functions 3 §1. Nature of Functions. 3. Their Classification. 3. Func- tional Notation. 4. Functions of Several Variables. 5. Func- tions of Functions. 6. Product of the First n Numbers. 7. Bi- nomial CoeflBcients. 8. Graphic Representation of Functions. 9. Continuity and Discontinuity of Functions. 10. Many- valued Functions. Chaptee II. Of Limits and Infinitesimals 17 § 11. Limits. 12. Infinites and Infinitesimals. 13. Properties. 14. Orders of Infinitesimals. 15. Orders of Infinites. Chaptek III. Of Differentials and Dekivatives 35 § 16. Increments of Variables. 17. First Idea of DifEerentials and Derivatives. 18. Illustrations. 19. Illustration by Velocities. 20. Geometrical Illustration. Chapter IV. Differentiation of Explicit Functions 31 § 21. The Process of Differentiation in General. 32. DifEeren- tials of Sums. 33. Differential of a Multiple. 34. Differential of a Constant. 35. Differentials of Products and Powers. 36. Dif- ferential of a Quotient of Two Variables. 37. Differentials of Ir- rational Expressions. 38. Logarithmic Functions. 39. Expo- nential Functions. 30. The Trigonometric Functions. 31. Cir- cular Functions. 32. Logarithmic Differentiation. 33. Velocity or Derivative with Respect to the Time. Chapter V. Functions of Several Variables and Implit cit Functions 54 § 34. Partial Differentials and Derivatives. 35. Total Differen- tials. 36. Principles Involved in Partial Differentiation. 37. Dif- Vi CONTENTS. PASE fcrentiation of Implicit Functions. 38. Implicit Functions of Sev- eral Variables. 39. Case of Implicit Functions expressed by Simultaneous Equations. 40. Functions of Functions. 41. Func- tions of Variables, some of which are Functions of the Others. 43. Extension of the Principle. 43. Nomenclature of Partial Derivatives. 44. Dependence of the Derivative upon the Form of the Function. Chapter VI. DBRivATrvES of EiaHEB Orders 74 §45. Second Derivatives. 46. Derivatives of Any Order. 47. Special Forms of Derivatives of Circular and Exponential Functions. 48. Successive Derivatives of an Implicit Function. 49. Successive Derivatives of a Product. 50. Successive Deriva- tives with Respect to Several Equicrescent Variables. 51. Result of Successive Differentiations independent of the Order of the Difflerentiations. 53. Kotation for Powers of a Differential or Derivative. Chapter VII. Speciai Cases of Successivb Dbrivatites. . . 86 § 58. Successive Derivatives of a Power of a Derivative. 54. De- rivatives of Functions of Functions. 55. Change of the Equicres- cent Variable. 56. Two Variables connected by a Third. Chapter VIII. DBVBiiOPMENTS ts Series 95 § 57. Classification of Series. 58. Convergence and Divergence of Series. 59. Maclaurin's Theorem. 60. Ratio of the Circum- ference of a Circle to its Diameter. 61. Use of Symbolic Nota- tion for Derivatives^ 63. Taylor's Theorem. 63. Identity of Taylor's and Maclaurin's Theorems. 64. Cases of Failure of Taylor's and Maclaurin's Theorems. 65. Extension of Taylor's Theorem to Functions of Several Variables. 66. Hyperbolic Functions. Chapter IX. Maxima aud Minima of Functions of a Sin- gle Variable 117 §67. Definition of Maximum Value and Minimum Value. 68. Method of finding Maximum and Minimum Values of a Func- tion. 69. Case when the Function which is to be a Maximum or Minimum is expressed as a Function of Two or More Variables connected by Equations of Condition. Chapter X. Indbtbrminatb Forms 128 § 70. Examples of Indeterminate Forms. 71. Evaluation of the Form -r-. 73. Forms — and X oo . 73. Form oo — oo . oo 75. Forms 0° and co ". CONTENTS. VU PASS Chapter XI. Of Platte Oubtbs 137 § 76. Forms of the Equations of Curves. 77. Inflnitesimal Ele- ments of Curves. 78. Properties of Infinitesimal Ares and Chords. 79. Expressions for Elements of Curves. 80. Equa- tions of Certain Noteworthy Curves. The Cycloid. 81. The Lemniscate. 83. The Archimedean Spiral. 83. The Logarith- mic Spiral. Chaptbb XII. Tangents and Nokmais 147 §84 Tangent and Normal compared with Subtangent and Subnormal. 85. General Equation for a Tangent. 86. Sub- tangent and Subnormal. 87. Modified Forms of the Equation. 88. Tangents and Normals to the Conic Sections. 89. Length of the Perpendicular from the Origin upon a Tangent or Noi-mal. 90. Tangent and Normal in Polar Co-ordinates. 91. Perpendicular from the Pole upon the Tangent or Normal. 93. Equation of Tangent and Normal derived from Polar Equation of the Curve. Chapter XIII. Of Asymptotes, Singulab Pointb and Clkve-teacing 157 §93. Asymptotes. 94. Examples of Asymptotes. 95. Points of Inflection. 96. Singular Points of Curves. 97. Condition of Singular Points. 98. Examples of Double-points. 99. Curve- tracing. Chaptbb XIV. Thbokt of Envelopes 169 §100. Envelope of a Family of Lines. 101. All Lines of a Family tangent to the Envelope. 103. Examples and Applications. Chapter XV. Of Cuetatube, Etoltjtes and Involutes 180 §103. Position; Direction; Curvature. 104. Contacts of Differ- ent Orders. 105. Intersection or Non-intersection of Curves ac- cording to the Order of Contact. 106. Radius of Curvature. 107. The Osculating Circle. 108. Radius of Curvature when the Abscissa is not taken as the Independent Variable. 109. Ra- dius of Curvature of a Curve referred to Polar Co-ordinates. 110. Evolutes and Involutes. 111. Case of an Auxiliary Variable. 113. The Evolute of the Parabola. 113. Evolute of the Ellipse. 114. Evolute of the Cycloid. 115. Fundamental Properties of the Evolute. 116. Involutes. viu CONTENTS. PAET II. THE INTEGRAL CAL0ULU8. PAOK Chapteb I. The Elbmbntaby Forms of Intbgkation 201 §117. Deflnition of Integration. 118. Arbitrary Constant of Integration. 119. Integration of Entire Functions. 120. The Logarithmic Function. 131. Another Method of obtaining the Logarithmic Integral. 123. Exponential Functions. 123. The Elementary Forms of Integration. Chapter II. Integrals immbdiatelt reducible to the Elementary Forms 209 § 124. Integrals reducible to the Form / ydy. 125. Appli- cation to the Case of a Falling Body. 136. Reduction to the Loga- rithmic Form. 127. Trigonometric Forms. 138. Integration of a^ "^^ ¥^r 129- Integrals of the Form f^~^. 130. Inverse Sines and Cosines as Integrals. 131. Two Forms of Integrals expressed by Circular Functions. 133. Integration of 133. Integration of . 134. Exponen- tial Forms. Chapter III. Integration by Rational Transformations. . 333 § 135. Integration of ^ ' —dx, - — j-t-t- and a!» ' (a + Ja;)" a + fee ± «'' 136. Reduction of Rational Fractions in general. 137. Integra- tion by Parts. Chapter IV. Integration of Irrational Algebraic Dif- ferentials 333 §138. When Fractional Powers of the Independent Variable enter into the Expression. 139. Cases when the Given Dififeren- tial Contains an Irrational Quantity of the Form 4/a + fes + esfl. cLt 140. Integration of dB = — 141. General Theory r V (zr* -}-br — 1 of Irrational Binomial Differentials. 143. Special Cases when TO + 1 = ra, OT m + l + np = — n. 143. Forms of Reduction of Irrational Binomials. 144. Formulae A and B, in which m is increased or diminished by n. 145. Ponnulae C and D, in which p is increased or diminished by 1. 146. Effect of the Formulae. 147. Case of Failure in this Reduction. CONTENTS. ix PAGE Chaptek v. Integration of Tkanscendbnt Ftihctions 246 § 148. Integration of / &^ cos nxdx and / Cmx sin vaxla!. 149. Integration of sin" » cos" xd/x. 150. Special Cases of / sin"" x co^xdx. 151. Integration of „ . „ — , — = 5-. 152. Inteara- m^ sin^ x-\-n? cos-' x " tion of — —^ . 153. Special Cases of the Last Two Forms. a -\- cos y 154. Integration of sin mx cos nxdx. 155. Integration by Devel- opment in Series. Chaptbk VI. Op Definite Integrals 255 §156. Successive Increments of a Variable. 157. Differential of an Area. 158. The Formation of a Definite Integral. 159. Two Conceptions of a Definite Integral. 160. Differentiation of a Definite Integral with respect to its Limits. 161. Examples and Exercises in finding Definite Integrals. 162. Failure of the Method when the Function becomes Infinite. 163. Change of Variable in Definite Integrals. 164. Subdivision of a Definite In- tegral. 165. Definite Integrals through Integration by Parts. Chapter VII. Successive Intesbation 372 § 166. DLfferentiation under the Sign of Integration. 167. Ap- plication of the Principle to Definite Integrals. 168. Integration by means of Differentiating Known Integrals. 169. Application to a Special Case. 170. Double Integrals. 171. Value of a Func- tion of Two Variables obtained from its Second Derivative. 172. Triple and Multiple Integrals. 173. Definite Double Inte- grals. 174. Definite Triple and Multiple Integrals. 175. Product of e-x'dx. Chapter VIII. Ebctification and Quadrature 285 §177. The Eectification of Curves. 178. The Parabola. 179. The Ellipse. 180. The Cycloid. 181. The Archimedean Spiral. 183. The Logarithmic Spiral. 183. The Quadrature of Plane Figures. 184. The Parabola. 185. The Circle and the Ellipse. 186. The Hyperbola. 187. The Lemniscate. 188. The Cycloid. Chapter IX. The Cubatubb of Volumes 297 §189. General Formulae. 190. The Sphere. 191. The Pyra- mid. 192. The Ellipsoid. 193. Volume of any Solid of Revolu- tion. 194. The Paraboloid of Revolution. 195. The Volume gen- erated by the Revolution of a Cycloid around its Base. 196. The Hyperboloid of Revolution of Two Nappes. 197. Ring-shaped Solids of Revolution. 198. Application to the Circular Ring. 199. Quadrature of Surfaces of Revolution. 200. Examples of Surfaces of Revolution. PART I. THE DIFFERENTIAL CALCULUS. USE OP THE SYMBOL =. The symbol = of identity as employed in this work indi- cates that the single letter on one side of it is used to repre- sent the expression or thing defined on the other side of it. When the single letter precedes the symbol =, the latter may commonly be read is put for, or is defined as. When the single letter follows the symbol, the latter may be read which let us call. In each case the equality of the quantities on each side of H does not follow from anything that precedes, bat is assumed at the moment. But haying once made this assumption, any equations which may flow from it are expressed by the sign =, as usual. PART I. THE DIFFERENTIAL CALCULUS. CHAPTER I. OF VARIABLES AND FUNCTIONS. 1. In the higher mathematics we conceive ourselves to be dealing with pairs of quantities so related that the value of one depends upon that of the other. For each value which we assign to one we conceive that there is a corresponding value of the other. For example, the time required to perform a journey is a function of the distance to be travelled, because, other things being equal, the time varies when the distance varies. We study the relation between two such quantities by as- signing values at pleasure to one, and ascertaining and com- paring the corresponding values of the other. The quantity to which we assign values at pleasure is called the independent variable. The quantity whose values depend upon those of the inde- pendent variable is called a function of that variable. Example I. If a train travels at the rate of 30 miles an hour, and if we ask how long it will take the train to travel 15 miles, 30 miles, 60 miles, 900 miles, etc., we shall have for the corresponding times, or functions of the distances, half an hour, one hour, two hours, thirty hours, etc. 4 THE DIFFERENTIAL CALCULUS. In thinking thus we consider the distance to be travelled as the independent variable, and the time as the function of the distance. Example II. If between the quantities x and y we have the equation y = 3fla;', we may suppose a; = -l, 0, +1, +2, +3yetc., and we shall then have y = 2a, 0, 3«, 8a, 18«, etc. Here x is taken as the independent variable, and y as the function of x. For each value we assign to x there is a corre- sponding value of y. When the relation between the two quantities is expressed by means of an equation between symbolic expressions, the one is called an analytic fnuction of the other. An analytic function is said to be Explicit when the symbol which represents it stands alone on one side of the equation; Implicit when it does not so stand alone. Example. In the above equation y is an explicit function of X. But if we have the equation y''-\-xy = x\ then for each value of x there will be a certain value of y, which will be found by solving the equation, considering y as the unknown quantity. Here y is still a function of x, be- cause to each value of x corresponds a certain value of y; but because y does not stand alone on one side of the equation it is called an implicit function. Remabk. The difference between explicit and implicit functions is merely one of form, arising from the different ways in which the relation may be expressed. Thus in the two forms VARIABLES AND FUNCTIOm. 5 y — %ax'', y — 2ax'' = 0; y is the same function of x; but its form is explicit in the first and implicit in the second. An implicit function may be reduced to an explicit one by solving the equation, regarding the function as the unknown quantity. But as the solution may be either impracticable or too complicated for conyenient use, it may be impossible to express the function otherwise than in an implicit form. 3. Classification of Functions. When y is an explicit function of x it is, by definition, equal to a symbolic expression containing the symbol x. Hence we may call either y or the symbolic expression the function of x, the two being equiva- lent. Indeed any algebraic expression containing a symbol is, by definition, a function of the quantity represented by the symbol, because its value must depend upon that of the sym- bol. Every algebraic expression indicates that certain operations are to be performed upon the quantities represented by the symbols. These operations are: 1. Addition and subtraction, included algebraically in one class. 3. Multiplication, including involution, 3. Division. 4. Evolution, or the extraction of roots. A function which involves only these four operations is called algebraic. Functions are classified according to the operations which must be performed in order to obtain their values from the values of the independent variables upon which they depend. A rational function is one in which the only operations indicated upon or with the independent variable are those of addition, multiplication, or division. 6 THE DIFFERENTIAL CALCULUS. An entire function is a rational one in which the only in- dicated operations are those of addition and multiplication. Examples. The expression fls + Ja; + ca;' + dx' is an entire function of x, as weU as of a, i, c and d. The expression , m . c X x" -\-nx is a rational function of x, but not an entire function of x. An irrational function of a Tariable is one in which the extraction of some root of an expression containing that vari- able is indicated. Example. The expressions V'fls + bx, (a + mx' + «a;') are irrational functions of x. Functions which cannot be represented by any finite com- bination of the algebraic operations above enumerated are called transcendental. An exponential function is one in which the variable enters into an exponent. Example. The expressions {a + a;)"", d" are entire functions of x when n and y are integers. But they are exponential functions of y. Other transcendental functions are: Trigonometric functions, the sine, cosine, etc. IiOgarithmic functions, which require the finding of a logarithm. Circular functions, which are the inverse of the trigo- nometric functions; for example, if y = a trigonometric function of x, sin x for instance, then a; is a circular function of y, namely, the arc of which y is the sine. . , , •? VARIABLES AND FUNCTIONS. 7 3. Functional Notation. For brevity and generality we may represent any function of a variable by a single symbol having a mark to indicate the variable attached to it, in any form we may elect. Such a symbol is called a functional symbol or a symbol of operation. The most common functional symbols are F, f and 0; but any signs or mode of writing whatever may be used. Then, such expressions as F{^), A^), 'K^), each mean " some symbolic expression containing x." The variable is enclosed in parentheses in order that the function may not be mistaken for the product of a quantity F,foT 0by a;. Identical Functions. Functions which indicate identical operations upon two variables are considered as identical. Example. If we consider the expression a + ly as a certain function of y, then a-\-hx is that same function of x, and a + b{x + y) is that same function olx-\- y. When the functional notation is applied, then: Identical functions are represented by the same functional symbols. Examples. If we put F(x) = a + bx, we shall have F(y) = a + by; nf) = « + h"; F{x' -f) = a + i{x' - f). 8 THE DIFFERENTIAL OALGULUS. In general. If we define afunctional symbol as representing a certain function of a variable, that same symbol applied to a second variable will represent the expression formed by sub- stituting the second variable for the first. In applying tWs rule any expression may be regarded as a Tariable to be substituted, as, in the last example, we used x' — y' as a variable to be substituted for x in the original expression. EXERCISES. 1. If we put 0(a;) = ax', it is required to form and reduce the functions 0(y), 0(5), 0(a), <P{-x), 4>{x'), 0(1). a. Putting _, , 1 + a; it is required to form and reduce ^«+')- ^(5). ^(|. ^(1). ^(l)+'(^)- 3. Putting it is required to form and reduce /(.-.), /(. + .), /(I), /(£). 4. If 0(a;) = a'x + ex', form and reduce the expressions <p{x'), <p{a'), <p{ax), (P{bx), (f>{a + c), fp{a-c). 5. Suppose 0(a;) = ax' — a'x, and thence form <P{y)> <P{x), (fr{by), <P{^ + y)> 'P{.x + a), <p{x — a), (p{x + ay), (f){x - az/), ct>{x'). VARIABLES AND FVNOTIOm. 9 6. Suppose /(a;) = x', and thence form the values of /(I). A^l, A^\ A^% A^% /(«-•")• t. Let us put 0(ot) = m{m — l){m — 2) (m — 3); thence find the values of 0(6), 0(5), 0(4), 0(3), 0(3), 0(1), 0(0), 0(- 1), 0(- 2). 8. Prove that if we put (j){x) = a^, we shall have 0(a; + 2/) = 0(a:) x 0(2^); 0(a;2^) = [0(2;)]" =- [0(2/)]*- 4. Functions of Several Variables, An algebraic expres- sion containing several quantities may be represented by any symbol having the letters which represent the quantities at- tached. Examples. We may put 0(a;, y) = ax — ly, the comma being inserted between x and y so that their product shall not be understood. We shall then have 0(m, n) = am — in, 4>{y, ») = ay — ix, the letters being simply interchangedj 0(a; -\- y, X - y) = a{x + y) - i{x - y) = {a- b)x +(a + b)y; 0(a, b) = a' - b'l 0(5, a) = ab — ba = 0; 0(a + b, ab) = a{a -{- b) — ab'; 0(«, a) = a' — la; etc. etc. If we put 0(a, b, c) = 3a + 35 — 5c, we shall have 0(a;, z, y)=2x + 3z- by; <l)(z, y, x)=%z-\-^ - 5x; <j>{m, m, — m) = 2m -\- 3m -\- 5m = 10m; 0(3, 8, 6) = 3-3 + 3-8 - 5-6 = 0. 10 THE nrFFERENTIAL CALCULUS. EXERCISES. Let us put <l>{x, y) =3z — 4:y; f{x, y) = ax + by; f(x, y, z)=ax + by — abz. Thence form the expressions: I- fPiy, ^)- 2- 0K i)' 3' 0(3, 4). 4. 0(4, 3). 5. 0(10, 1). 6. /(a, 5). 7. Ah a). 8. /(y, a;). 9. /(7, - 3). lO- A<1> —P)- "• /(2, !»» y)- 12- /(*, «J 2). 13- /(«, 5, c)- 14- /(«', 5°, c')- 15- A-a,-i,-ab). Sometimes there is no need of any functional symbol except the parentheses. For example, the form (m, n) may be used to indicate any function of m and n. EXERCISES. T J. 4. / \ "»(»* — l){m — 3) Let us put (to, n) = —) -{-f ^, ^ ' ' n{n — V)(n — 2) then find the values of — 1. (3, 3). 2. (4, 3). 3. (5, 3). 4- (6, 3). 5. (7, 3). 6. (8, 3). 7. (3, - 1). 8. (3, - 3). 9. (4, - 2). 5. Functions of Functions. By the definitions of the pre- ceding chapter, the expression f[<P{^)) will mean the expression obtained by substituting 0(a;) for x in /(a;). We may here omit the larger parentheses and write /0(a;) instead of /( 0(a;) ) . For example, using the notation of exercises 1 and 3 of § 3, we shall have ,,, . ax" — a a;' — 1 ax' + a x' + V VARIABLES AND FUNCTIONS. 11 For brevity we use the notation tp^x) = <p{<t>{x)y Continuing the same system, we have ct>\x) = cl,[ct>\x)^ =0=(0W); etc. etc. etc. EXAMPLI IS. 1. If cl){x) = ax\ en cjy'ix) = a(axy = a'x'; <p\x) = a{a'xy = a'x'; etc. etc. etc. 3. If f{x) = a-x, en f'(x) = a — (a — x) = a f\x) = a — fix) = a — x; and, in general, Eemakk. The functional nomenclature may be simplified to any extent. 1. The parentheses are quite unnecessary when there is no danger of mistaking the form for a product. 2. When it is once known what the variables are, we may write the functional symbol without them. Thus the symbol may be taken to mean <px or <p{x). 6. Product of the First n Numbers. The symbol n\, called factorial n, is used to express the product of the first n num- bers, 1-2-3-...M. Thus, 1! = 1; 2! = l-2 = 2; 3! = l-2-3 = 6; 4! = l-3-3-4^24; etc. etc. 12 THE DIFFERENTIAL CALCULUS. It will be seen that 2! = 2-1!; 3! = 3-21; and, in general, n\ — n-(n — V)\, whatever number n may represent. EXERCISES. Compute the values of — I. 5! 2. 6! 3. 8! 7! 8! ^' 3! 4! ^' 3! 5! 6. Prove the equation 2 • 4 • 6 • 8 • . . . 2m = 2"w ! 7. Prove that, when n is even, Mj _ n{n — 2) (w —4)... 4-2 2' ~ » 7 . Binomial Coefficients. The binomial coefiBcient w(w — 1) (m — 2) to s terms 1-2-3-...S is expressed in the abbreviated form (7). the parentheses being used to distinguish the expression from the fraction — . s EXAMPLES. 3. 7- 6-5-4-3 2-3-4-5 ©4= In\ n In\ _ n{n — 1) (w — 2) \3/ "" 1-2-3 VARIABLES AND FVW0TI0N8. 13 EXERCISES. ProTe the formulse: I) = (I)- 5\ _ 5! 2/ ~%\ 3]" '• ^%)=[^^- n\ n-\-l n-\-l fn .5 + 1/ s + 1 n\ . In \ tn -\- 1 s\ {n — s)\ 6. (!) + (!)=/« + ' r) + (l) = (- i)+f )■ 8. Grapliic Representation of Functions. The methods of Analytic Geometry enable us to represent functions to the eye by means of curves. The common way of doing this is to represent the independent variable by the abscissa of a point, and the corresponding value of the function by its ordinate. Let a;,, a-,, x^, etc., be different values of the in- dependent variable, and y^> y,> y.> etc., the cor- responding values of the function. We lay off upon the axis of abscis- sas the lengths OX^, T y (L Pl-'-T- Pa y^ Vi JCi X2 Xs ox„ ox, to X^, X,, FlQ. 1. 1' 3, etc., equal X,, etc., and terminating at the points X^, X„ X„ etc. At each of these points we erect a perpendicular to rep- resent the corresponding value of y. The ends, P,, P^, P^, of these perpendiculars will generally terminate on a curve line, the form of which shows the nature of the function. It must be clearly seen and remembered that it is not the curve itself which represents the values of the function, but the ordinates of the curve. 14 THE DIFFERENTIAL CALCULUS. Fia. 3. 9. Continuity and Discontinuity of Fnrwtions. Let us consider the graphic repreeentation of a function in the most geai€Tal"way. "We measure off a series of values, OX,, 0X„ 0X3, etc., of the independent variable, and at the points X„ X,, X3, etc., we erect ordinates. In order that the variable ordinate ^^ may actually be a function of x it is sufficient if, for every value of the abscissa, there is a corresponding value of the ordinate. N"ow we might conceive of such a function that there should be no • '' ' 1 ' ' 1 1 ' ' ' 1 ' 1 ' ' '^f- relation between the different val- ues of the ordinates, but that every separate point should have its own separate ordinate, as shown in Fig. 2. If this remained true how numerous soever we made the ordi- nates, then the ends of the latter would not terminate in any curve at all, but would be scattered over the plane. Such a function would be called discontinuous at every point. Such, however, is not the kind of functions commonly considered in mathematics. The functions with which we are now concerned are such that, however irregular they may appear when the values of x are widely separated, the ends of the ordinates will terminate in a curve when we bring those values close enough together. If a function is such that when the point representing the independent variable moves continuously from X, to X, (Fig. 1) the end of the ordinate describes an unbroken curve, then we call the function continuous between the values a;, and a;, of the independent variable. If the curve remains unbroken how far soever we suppose a; to increase, positively or negatively, we call the function continuous for all values of the independent variable. VABIABLB8 AND FUNCTIONS. 15 But if there is a value a oix for which there is a break of any kind in the curve, we call the function discontinuous for i]ie value a of the independent variaMe. Let us, for example, consider the funjetioji 6(a — x)' Let us measure ofl on the axis of abscissas the length OX = a . Then as we make our varying ordinate approach X from the left it will increase positively without limit, and the curve will extend upwards to infinity; if we approach X from the right-hand side, the ordinate will be negative and the curve will go downwards to infinity. Thus the curve will not form a continuous branch from the one side to the other. Thus the above function is discontinuous for the value a of x. / X Fio. 3. 10. Many-valued Functions. In all that precedes, we have spoken as if to each value of the independent variable corresponded only one value of the function. But it may 16 THE DIFFEBMNTIAL CALGULUS. happen that there are several such values. For example, if y is an implicit function of x represented by the equation y' + mxy^ + nx'y -\-px^ — 0, then we know, by the theory of equations, that there tyill be three values of y for each value assigned to the variable x. Def. According as a function admits of one, two or n values, it is called one-valued, two-valued or w-valued. Infinitely-valued Functions. It piay happen that to each value of the variable there are an infinity of different values of the function. A case of this is the function sin <" '' x, or the arc of which x is the sine. This arc may be either the smallest arc which has x for its sine, or this smallest arc in- creased by any entire number of circumferences. Take, for example, the arc whose sine shall be+i The two smallest arcs will be 30° = \7t and 150° = ^n. But if we take the function in its most gen- eral sense it may have any of the values {%+\)7t; (4 + i);r; (6 + i);r, etc., or (2+f);r; (4 + |);r; (6 + |);r, etc. When we represent an m-valued function graphically, there will be n values to each ordi- nate. Hence each ordinate will cut the curve in n points, real or imaginary. The figure in the margin represents the infi- nitely-valued function « = a sin ^~"— . When — a <.x < -\-a, any ordinate will cut ihe curve in an infinity of points. LIMITS AND INFINITESIMALS. 17 CHAPTER II. OF LIMITS AND INFINITESIMALS. 1 1 . Limits. The method of limits is an indirect method of arriving at the values of certain quantities which do not admit of direct determination. The method rests upon the following axioms and definition: Axiom I. Any quantity, however small, may be multiplied by so great a number as to exceed any other quantity of the same kind, however great, to which a fixed value is assigned. Axiom II. Conversely, any quantity, however great, may be divided into so many parts that each part shall be less than any other quantity of the same kind, however small, to which a fixed value is assigned. Axiom III. Any quantity may be divided into any num- ber of parts ; or multiplied any number of times. Def. The limit of a variable quantity X is a quantity L, which we conceive Xto approach in such a way that the dif- ference L — X becomes less than any quantity we can name, but which we do not conceive X to reach. Example. If we have a variable quantity X and a con- stant quantity L, and if X, in varying according to any mathe- matical laWj takes the successive values L ± 0.1, L ± 0.01, L ± 0.001, L ± 0.0001, and so on indefinitely, without becoming equal to L, then we say that L is the limit of x. 18 THE DIFFERENTIAL CALCULUS. 13. Infinites and Infinitesimals. Definitions. 1. An infinite quantity is one considered as becoming greater than any quantity which we can name. 2. An infinitesimal quantity is one considered in the act of becoming less than any quantity which we can name; that is, in the act of approaching zero as a limit. 3. A finite quantity is one which is neither infinite nor in- finitesimal.* Examples. If of a quantity x we either suppose or prove X > 10, X > 100, X > 100000, and so on without end, then x is called an infinite quantity. If of a quantity h we either suppose or prove h < 0.1, h < 0.001, h < 0.00001, and so on without end, then h is an infinitesimal quantity. The preceding conceptions of limits, infinites and infinitesi- mals are applied in the following ways: Let us have an inde- pendent variable x, and a function of that variable which we call y. Now, in order to apply the method of limits, we may make three suppositions respecting the value of x, namely: 1. That X approaches some finite limit. 2. That X increases without limit (i.e., is infinite). 3. That X diminishes without limit (i.e., is infinitesimal). In each of these cases the result may be that y approaches a finite limit, or is infinite, or is infinitesimal. * Strictly speaking, the words infinite and infinitesimal are both adjec- tives qualifying a qiuintity. But the second has lately heen used also aa a noun, and we shall therefore use the word infinite as a noun meaning infinite quantity. LIMITS AND INFINITESIMALS. 19 For example, let us have x-\-a y = — ■ — • " X — a Then— When X approaches the limit a, y becomes infinite. 'When X becomes infinite, y approaches the limit + !• When X becomes infinitesimal, y approaches the limit — 1. The symbol i, followed by that of zero or a finite quantity, means ''approaches the limit." The symbols ioo mean "increases without limit" or "becomes infinite." Hence the three last statements may be expressed symbolically, as follows: x-\- a , When X i a, then X — a When a; = 00, then == + 1: X — a etc. etc. The same statements are more commonly expressed thus: ,. x-\- a . . . lim. — ■ — (x = a) = 00 : X — a^ ' lim. — ■ — (a; = 00 ) = +1: X — a^ ' lim. 5+L? (^ ^ 0) = - 1. X — a^ ' 13. Properties of Infinite and Infinitesimal Quantities. Theoebm I. The product of an infinitesimal by any finite factor, however great, is an infinitesimal. Proof. Let /* be the infinitesimal, and n the finite factor by which it is multiplied. I say how great soever n may be, 7ih is also an infinitesimal. For, if tih does not become less than any quantity we can name, let a be a quantity less than which it does not become. Then if we take, as we may, h < ~, (Axiom III.) n ^ ' we shall have nh < a. 20 THE DIFFERENTIAL CALCULUS. That is, nh is less than a and not less than a, which is absurd. Hence oih becomes less than any quantity we can name, and is therefore infinitesimal, by definition. Theoeem II. The quotient of an injitiite quantity hy any finite divisor, hotoever great, is infinite. Proof. Let X be the infinite quantity, and n the finite divisor. It X -i- n does not increase beyond every limit, let K be some quantity which it cannot exceed. Then by taking • X>n£:, (Ax. III.) we shall have — > X; n that is, — greater than the quantity which it cannot exceed, which is absurd. Hence X -^ n increases beyond every limit we can name when X does, and is therefore infinite when X is infinite. Theoeem III. The product of any finite quantity, how- ever small, by an infinite multiplier, is infinite. This foUows at once from Axiom I., since by increasing the multiplier we may make the product greater than any quan- tity we can name. Theoeem IV. The quotient of any finite quantity, how- ever great, iy an infinite divisor is infinitesimal. This follows at once from Axiom II., since by increasing the divisor the quotient may be made less than any finite quantity. Theoeem V. The reciprocal of an infinitesimal is an in- finite, and vice versa. Let h be an infinitesimal. If :r is not infinite, there must A be some quantity which we can name which t- does not ex- LIMITS AND INFINITESIMALB. 21 ceed. Let K\i& that quantity. Because h is infinitesimal, we may have which gives j-> K; that is, 7- greater than a quantity it can never exceed, which is absurd. The converse theorem may be proved in the same way. 14. Orders of Infinitesimals. Def. If the ratio of one infinitesimal to another approaches a finite limit, they are called infinitesimals of the same order. If the ratio is itself infinitesimal, the lesser infinitesimal is said to be of higher order than the other. Theorem VI. If we have a series proceeding according to the powers of h, A+Bh+Ch' + Dh' + etc., in which the coefficients A, B, G, are all finite, then, if h be- comes infinitesimal, each term after the first is an infinitesi- mal of higher order than the term preceding. Proof. The ratio of two consecutive terms, the third and fourth for example, is Dh' _D G¥~ G ' D By hypothesis, Cand D are both finite; hence ^ is finite; hence when h approaches the limit zero, ^/j becomes an in^ finitesimal (§13, Th. I.). Thus, by definition, the term Dh' is an infinitesimal of higher order than G¥. Def. The orders of infinitesimals are numbered by taking some one infinitesimal as a base and calling it an infinitesi- mal of the first order. Then, an infinitesimal whose ratio to 22 THM DIFFERENTIAL CALCULUS. the wth power of the base approaches a finite limit is called an infiiiitesimal of the nth order. Example. If A be taken as the base, the term Bh is of the first order ' .■ Bh:h — the finite quantity B; CW " " second" • .■ Ch' : h' = " " C; Eli^ " " ?tth " • . • Eh"" : A" = " " E. Cor. 1. Since when « = we have BJi^ = Bh" = B for all values of h, it follows that an infinitesimal of the order zero is the same as a finite quantity. Cor. 2. It may be shown in the same way that the product of any two infinitesimals of the first order is an infinitesimal of the second order. 15. Orders of Infinites. If the ratio of two infinite quantities approaches a finite limit, they are called infinites of the same order. If the ratio increases without limit, the greater term of the ratio is called an infinite of higher order than the other. Theorem VIL In a series of terms arranged according to the powers of x, A+Bx+Ox' + Dx' + etc., if A, B, C, etc., are all finite, then, when x becomes infinite, each term after the first is an infinite of higher order than the term preceding. For, the ratio of two consecutive terms is of the form -^.x, x> which becomes infinite with x (Th. III.). Def. Orders of infinity are numbered by taking some one infinite as a base, and calling it an infinite of the first order. Then, an infinite whose ratio to the nth. power of the base approaches a finite limit is called an infinite of the «th order. Thus, taking x as the standard, when it becomes infinite we call Bx infinite of the first order, Gx' qI the second order, etc. LIMITS AND INFINITESIMALS. 23 NOTE ON THE PRECEDING CHAPTERS. In beginning the Calculus, conceptions are presented to the student which seem beyond his grasp, and methods which seem to lack rigor. Really, however, the fundamental principle of these methods is as old as Euclid, and is met with in all works on elementary geometry which treat of the area of the circle. The simplest form in which the princi- ple appears is seen in the following case. Let us have to compare two quantities A and B, in order to determine whether they are equal. If they are not equal, then they must differ by some quantity. If, now, taking any arbitrary quantity Ti, we can prove that A-B<h wUJumt making a/ny supposition respecting the valiis of h, this will show that A and B are rigorously equal. For if they differed by the quantity a, then when /i was less than a the above inequality would not hold true. But as we have been supposed to prove it for all vnlues of h, it must be true when h is less than a. In this case h might be considered an infinitesimal, although in the Elements of Euclid it is represented on the page of the book by a figure nearly an inch square. Infinitesimal quantities were formerly called infinitely small. When they were introduced by Leibnitz many able mathematicians were unable to accept them. Bishop Berkeley wrote several essays against them, in one of which he suggested that they might be called the ghosts of departed quantities. The following propositions are presented in the hope that they may save the student unnecessary efforts of thought in the study of this subject. Firstly, there is no need that a quantity should be considered as ab- solutely infinite. A mathematical magnitude, considered as a quantity, must in its very nature have boundaries, because mathematics is con- cerned with the relation between magnitudes as greater or less, and we can compare two magnitudes as greater or less only by comparing their boundaries. An absolutely infinite magnitude, having no boun- daries to compare, cannot be compared with anything. Secondly, it is equally unnecessary to suppose the existence, either in nature or in thought, of quantities which are absolutely smaller than any finite quantity whatever. 24 THE DIFFERBNTIAL CALCULUS. But however small a quantity may be, there may always be another still smaller in any ratio. Hence, although it is perfectly true that no quantity can be otherwise than finite, yet it is equally true that a quantity may be less or greater than any fixed quantity we may name. Both infinite and infinitesimal quantities are therefore essentially in- definite, because by considering them in the act of increasing beyond, or decreasing below, every assignable value, we do away with the very pos- sibility of assigning values to them. They are used only as auxiliaries to lead us to a knowledge of finite quantities, and their magnitudes are never themselves the object of investigation. The essentially indefinite nature of infinites and infinitesimals may be illustrated as follows: If we have an equation of the form ct. then for every pair of finite values we assign to a and b there will be a definite value of x. But if we suppose A and S to be infinite, and at the same time inde- pendent of each other, there will be no definite value to x. Considering both terms as absolutely infinite, they will have no bounds, and there- fore cannot be compared in value. Considered as increasing without limit, one may be any number of times greater than the other, and thus the fraction may have any value we choose to assign it. Seeking for the value of such a fraction is like trying to answer the old question concerning the effect of an irresistible force acting upon an immovable obstacle. DIFFEBENTIALa AND DEBIVATI7M8. 25 CHAPTER III. OF DIFFERENTIALS AND DERIVATIVES. 16. Def. An increment of a variable is the difference between two values of that variable. An equivalent definition is: An increment is a quantity added to one value of a variable in order to obtain another value. Notation. An increment is expressed by the symbol A written before the symbol of the variable. Example. If we have the different variables X, y, u, and the increments Ax, Ay, Au, other values of the variables will be X + ^x, y + Ay, u + Au. Here A is not a factor multiplying x, but a symbol meaning "increment of," or, in familiar language, "a little piece of." In considering the respective increments of an independent variable, and of its function, the following five quantities come into play and are each to be clearly conceived. 1. A value of the independent variable, which we may take at pleasure. 2. The corresponding value of the function, which will be fixed by that of the independent variable. 3. An increment of the independent variable, also taken at pleasure. 4. The corresponding increment of the function, deter- mined by that of the independent variable. 5. The ratio of these increments. 26 THE DIFFERENTIAL CALCULUS. To represent these quantities, let the relation between the variable x and the function y be expressed by a curve. Let OX be one value of x, and OX' another. Let XP and X'P' be the corresponding values of y, leading to the points P and P' of the curve. We shall then have — 1. OX = X, a value of the independent variable. 2. XP = y, the corresponding value of the function. 3. XX = Ar, an arbitrary increment of x. 4. RP' = Ay, the corresponding increment of y. All 5. Then, by Plane Trigonometry, the quotient -~-_ will be the tangent of the angle PQX; that is, the tangent of the angle which the secant PP' makes with the axis of abscissas. Thus we have geometrical representations of the five fun- damental quantities under consideration. 17. First Idea of Differentials ajid Derivatives. Let us take, for illustration, the function y = nx'. (1) Giving to x the increment Ax, the new value of y will be 71 {x + Axf. Hence y -\- Ay = n{x + Ax)' = nx' -\- %nxAx -\- nAx', (2) DIFFBBBNTIAL8 AND DEBIVATIVES. 27 Subtracting (1) from (2), we have, for the increment of y, Ay = n{2x + Ax)Ax, (3) Because, when Ax becomes infinitesimal, lim, (3a; + Ax) = 2x, we hare, for the ratio of the increments, -£ = 2,ix + nAx, (4) and, when Ax becomes infinitesimal, lim. ^ = 'Hnx. (5) Def. The differential of a quantity is its infinitesimal increment; that is, its increment considered in the act of ap- proaching zero as its limit, or of becoming smaller than any quantity we can name. Notation. The differential of a quantity is indicated by the symbol d written before the symbol of the quantity. For example, the expressions dx, du, d(x + y), mean any infinitesimal increments of x, u, (x -\- y), respect- ively. Thus the substitution of d for A in the notation of incre- ments indicates that the increment represented by A is sup- posed to be infinitesimal, and that we are to consider the limit toward which some quantity arising from the increment then approaches. Using this notation, the equation (5) may be written -^-- — %nx. dx We also express this value of the limiting ratio in the form dy = 2nxdx; meaning thereby that the ratio of the two members of this equation has unity as its limit. This is evident from Eq. (3). 28 THE DIFFERENTIAL CALCULUS. Def. If « is a function of x, the ratio -~- of the differential " dx of y to that of x is called the derivative of the function, or the derived function. 18. Illustrations. As the logic of infinitesimals ofEera great difficulties to the beginner, some illustrations of the subject may be of value to him. Consider the following proposition: The error introduced hy neglecting all the powers of an in- crement above the first may be made as small as we please by diminishing the increment. Let us suppose n = %m the equation (1). We then have the equations y = 2a;'; Ay^i-cAx-\-%Ax'; _ ^^^ ^ = 4a; + 'HAx. The ratio of the two terms of the second member is %Ax Ax -^' '' W Let us now neglect this quantity and write the erroneous equation Ax If, now, we suppose Ax < Ax < Ax < X Too' X ioooo' X 1000000' etc.. the equation (b) will still be true within J_ 300 1 •^0000 1 2000000 etc. part; part; part; So long as we assign any definite value to Ax, it is clear that there will be some error in neglecting Ax. But there is no error in the equations dy = ixdx and — = 4a;, ctss DIFFERENTIALS AND DERIVATIVEB. 29 provided that we interpret them as expressing the limit which Ay -p approaches as Ax approaches the limit zero, and interpret all our results accordingly. 19. Illustration by Velocities. Let us consider what is meant by the familiar idea of a train which may be contin- ually changing its speed passing a certain point with a certain speed. To fix the ideas, suppose the train has just started and is every moment accelerating its speed in such manner that the total number of feet it has advanced is equal to the square of the number of seconds since it started. Put S = the distance travelled expressed in feet; t = the time expressed in seconds. We shall then have S = f, and for the distances travelled: Number of seconds, 0; 1; 2; 3; 4; 6; etc.; Distance travelled, 0; 1; 4; 9; 16; 35; etc.; Distance in each second, 1; 3; 5; 7; 9; 11; etc. B ill 8 I 5 I T I 9 1 11 I Fio. 6. Let this line represent the space travelled the first five seconds from the starting time, and let us inquire with what velocity the train passed the point B at the end of 4°. Since distance travelled = velocity x time, the mean ve- locity is found by dividing the space by the time required to pass over that space. Now, the train had travelled 16 feet in the time 4 seconds, and (4 + Aty feet in (4 + At) seconds, or 16 + 9,At + Af feet in (4 + At) seconds. Subtracting 16 feet and 4 seconds, we see that in the time At after the end of the 4 seconds the train went %At -\- At* = As feet. Hence its mean velocity from 4° to 4' + At is 30 THE DIFFERENTIAL OALCULUS. As At = (8 + At) feet per second. Now it is clear that, since the train was continually accel- erated how small soever we take At, the mean velocity during this interval will exceed that with which it passed B. But it is also clear that by supposing At to approach the limit zero, we shall approach the required velocity as our limit. Hence the velocity with which B was passed is rigorously ds di — 8 feet per second. Fig. 7. 30. Geometrical Illustration. If, in the figure, we sup- pose the point P' to approach P as its limit, the increments Ax and Ay will approach the limit zero, and the secant P'P will approach the tangent at the point P as its limit. We have already shown that Ax tangent of angle made by secant with axis of abscissas. Passing to the limit, we have the rigorous proposition -^ = tangent of angle which the tangent at the point P makes with the axis of abscissas. dx DIFFERBNTTATION OF EXPLICIT FUNOTIONB. 31 CHAPTER IV. DIFFERENTIATION OF EXPLICIT FUNCTIONS. 31. Def. The process of finding the differential and the derivative of a function is called differentiation. As exemplified in §§ 16, 17, it may be generalized as fol- lows: We have given (1) An independent variable = x. (2) A function of that variable = <t>{x). (3) We assign to x an increment = Ax; whereby ^{x) is changed into (p{x -\- Ax). (4) We thus have 0(a; -(- Ax) — (t>{x) as the increment of 4>{x). We may put A(j}{x) B 4){x + Ax) — <p(x). (5) We then form the ratio A(p{x) Ax («) and seek its limit when Ax becomes infinitesimal. Using the notation of the last chapter, we have flcf){x) ,. A<p{x) , . . „, which is the derivative of 0(a;). In order to find the ratio {a), it is necessary to develop cl){x -\- Ax) in powers of Ax to at least the first power of Ax. Let this development be 0(a; -f Ax) = X„ + X^Ax + X^Ax' + (1) In the second member of this equation X^, X„ etc., will be functions of x; and it is evident that X„ can be nothing but 32 TEB DIFFERENTIAL CALOXTLUS. (p{x) itself, because it is the value of <p{x + //») when Jx — 0. Thus we have J0(a;) = <p{x + Ax) - cl>(x) - (X, + XJx) Jx -\- . . . ; Passing to the limit, d<p{x) = X/lz; M£) = x, (3) Thus, by comparing with (1), we have the following: Theokem I. The derivative of a function is the coefficient of the first power of the increment of the independent variable when the function is developed in powers of that increment. If we have to differentiate a function of several variable quantities, x, y, z, etc., we assign an inprement to each vari- able, and develop the function in powers and products of the increments. Subtracting the original function, the remainder will be its increment. The terms of highest order in this increment, considered as infinitesimals, are then called the differential of the function. The following are the special cases by combining which all derivatives of rational functions may be found. 33. Differentials of Sums. Let x, y, z, u, etc., be any variables or functions whatever. Their sum will be x-\-y-\-z-\-u-\- etc. Assigning to each an increment, x will become x + Ax, y will become y -\- Ay, etc. Hence the sum will become X -\- Ax -\- y -\- Ay -\- z -\- Az -\- u -\~ Au -\- etc. Subtracting the original expression, we find the increment of the sum to be Ax + Ay -\- Az -\- Au -\- etc. DIFFEBENTIATIOW OF EXPLICIT FUNCTIONS. 33 Hence, when the increments become infinitesimal, d{x + y + ^ + w + etc.) = dx -{- dy -\- dz -\- du -\- etc., (3) or, in words: Theorem II. The differential of the sum of any number of variables is equal to the sum of their differentials. In this theorem the quantities x, y, z, u, etc., maybe either independent variables, or functions of one or more variables. 83. Differential of a Multiple. Let it be required to find the differential of ax, a being a constant. Giving X the increment Ax, the expression will become a{x -\- Ax). Then, proceeding as before, we find d{ax) = adx. (4) 34. Theorem III. The differential of any constatit is zero. For, by definition, a constant is a quantity which we sup- pose invariable, and to which we cannot, therefore, assign any increment. We therefore have, from Theorem I. when a; is a variable and a is a constant, d{x -|- a) = (?a; -)- = dx, or, in words: Theorem IV. The differential of the sum of a constant and a variable is equal to the differential of the variable alone. Eemark. It will be readily seen that the conclusions of §§ 23-24 are equally true whether we suppose the increments to be finite or infinitesimal. This is no longer the case when powers or products of some finite increments enter into the expression for other finite increments. 34 TSE DIFFERENTIAL CALCULUS. EXERCISBS. It is required, by combining the preceding processes, to form the difEerentials of the following expressions, supposing a, b and c to be constants, and all the other literal symbols to be variables. I. U — V. 2. 2u — V. 3- 5- 7- 9- V -\-x-\-c. a'x + b-y + c. iax -J- 5bx — y. Sx — a -\- ab. 4. ax -\- by. 6. 3a; + 4a«/ + h. 8. &bx — abc. 10. abx — abt. 11. 13- c{2x + a). ac{bu + «2;). 12. a{bx + ac). 14. 5c(2aa; — 3by). IS- x-y-z. 16. — aa; — by — cz. 17- 19. — a{bx — cy). X a' 18. - b{%ax - Zcv). 20. =^ + f-^ 21. (a + b + c){s + t + 3u- -%)• 22. {a + 2b' + 3c' . fau ^ \ c bv a + 2 si" 35. Differentials of Products and Powers. Take first the product of two variables, which we shall call u and v. Then Product = nv. Assigning the increments Ait, and Av, the product becomes (m + All) {v + Ji') = uv + «'^w + M-^« + AuAv. Subtracting the original function, nv, we find A{uv) — vAu + (?« + An) Av. Supposing the increments to become infinitesimals, the co- efficient of Av in the second member will approach u as its limit. Hence, passing to the limit (§ 14), d{uv) = vdu + udv. JDIFI'BBENTIATION OF EXPLICIT FUNCTIONS. 36 To extend the result to any number of factors, let P be the product of all the factors but one, and let the remaining fac- tor be X, so that we have Product = Px. By what precedes, we have d{Px) = xdP + Pdx. Supposing P to be a product of the two variables u and v, this result gives d{uvx) = xd{vii) + uvdx = vxdu -\- uxdv -\- uvdx. («) If we add a fourth factor, y, we shall have d{uvxy) — yd{uvx) -\- uvxdy. If we substitute for d{uvx) its value (a), we see that we pass frorn. the one case to the other by (1) multiplying all the terms of the first case by the common factor y; (3) adding the product of dy into all the other factors. We are thus led to the conclusion: Theoeem v. The differential of the product of any num- ler of variables is equal to the sum of the products formed by replacing each variable by its differential. Corollary. If the n factors are all equal, their product will become the wth power of the variable, and the n difEerentials will all become equal. Hence, when n is an integer, we have the general formula d^x^) = 2-"~VZa; + x'^~\lx -\- etc., to ?i terms, or dipf') = iix'^~'^dx. By combining the preceding processes we may form the differential of any entire function of any number of variables. EXAHPLES. I. d{ax + ixy -\- cxyz) = d{ax) + d{bxy) + d{cxyz) (Th. II. 32) = adx + bd(xy) + cd{xyz) (Th. III. 23) = adx -\- b{ydx -\- xdy) + c{yzdx -{- xzdy -j- xydz) = (a -\- by -\- cyz)dx + {bx + czz)dy + cxydz. 36 THE DIFFERENTIAL GALGULUS. 2. d{ax' + b) = d{ax') (Th. IV.) = ad{x') (§ 23) = Sax'dx. (Th. V., Cor.) 3. d{axY) = ad{xY) (§23) = a[i/"d{x') + «'%")] (Th. V.) = 3a«/ V Ja; + nax'y " - ^dy. (Th. V. , Cor. ) 4. d{a+ x')"- = n{a + a;=)"-'<?(a + x'')=2n{a + x')'—^xdx. EXERCISES. Form the differentials of the following expressions, suppos- ing the letters of the alphabet from a to ii to represent con- stants: 1. a-\-bx^ -{- ex*. Ans. {2bx -\-4tcx')dx. 2. B+Cy-{- Dy\ 3. axy. 4. bxyz. 5. a(x -\- yz). 6. a{x' + buv). 7. axy -\- buv. 8. h{x'y + xy'). 9. ax'y'. 10. Jx'y'*. II. aSa;'*/'' + Aw^z;"- 12. 2(«ia: + my). 13. (r + ^) (^ + ^r). 14. n{a — a;'). 15. aa;' — byz. 16. (« + 3;) (3 - 2/). 17. (a + x'){b-y'). 18. (a — a;) (« — a;'). 19. a;(a + a;) (5 — a;'). 20. (^ + 5a; + fe") (2/ + z). xy 21. (^+52/'+Cy)(fl2^+te) "• t- 23- (« + 5w*) (cx^ — ny''). X — uv 25. (a — x){b — a;')(c — x'). ' a . x-uv. , , 27. x{x' + y{a-x)\. 26. (M + W). a ^ ' 29. {ay' ~ bx') {x - y). ^ U bJ\a '^ b)' 32. «(«+a;)'. 33. (ff + a:?/)'. 34. (aa; + 5y)'. DIFFERENTIATION OF EXPLICIT FUNCTIONS. 37 36 Differential of a Quotient of Two Variables. Let the variables be x and y, and let q be their quotient. Then X and qy = x. Differentiating, we have ydq + qdy = dx. Solving so as to find the value of dq, _dx — qdy _ ydx — xdy ^ ~ y ~ ? ' Hence: Thbokbm VI. The differential of a fraction is equal to the denominator into the differential of the numerator, minus the numerator into the differential of the denominator, divided ly the square of the denominator. Eemakk. If the numerator is a constant, its difEerential vanishes, and we have the general formula d— — ,dx. X X EXERCISES. Form the differentials of the following expressions: X a -\- X 2. a + y a — X a -y a 1* a -\-l)X a + iy X + y « + : x' {b + yf m -\- nx^ 8. ;. m — nx lO. — 5-i — % X — y mx — ny^ 38 THE BIPFEBENTIAL CALCULUS. a -\- bx + ex' VI + xy '^' m - xY a , b a 17 19. xy + x'y ^' + ?/' 12. x + yz y + xz 14. 11 X X m n 16. x'~y' 1 1 18. X y a x'-y' -f x'+f 37. Differentials of Irrational Expression!!. Let it be re- quired to find the difEerential of the function m U = X", m and n being positive integers. Eaising both members of the equation to the wth power, we have «" = a;™. Taking the differentials of both members, nu"~^du = mx^^~^dx, whence du_mx'"~^_m re"'"' _vi a;*""' _m—-i dx~n v^"- ~ n /"^-"i ~ n "■" -^ ~ « ^ " ' ^"' yx"! X " a formula which corresponds to the corollary of Theorem V., where the exponent is entire. Next, let the fractional exponent be negative. Then _!1 1 U = X » = -— -, and, by Th. VI., d\x^l mx" dx in -JS-i , f^« = ^ =~ n ■ ^"~ "^ ~ w"* " '^'^■' X " a; » and, for the derivative, du _ m ---1 dx~~ n DIFFERENTIATION OF EXPLICIT FUNCTIONS. 39 From this equation and from {a) we conclude: Thboebm VII. The formula d{x'') = nx^'-^dx liolds true whether the exponent n is entire or fractional, posi- tive or negative. We thus derive the following rule for forming the differen- tials of irrational expressions: Express the indicated roots by fractional exponents, positive or negative, and then form the differential by the preceding methods. Examples. 1. dVa-\-x = d(a -A- a;)* = Ma -1- a;) - idx = —. — ; — r-. ^ ' ' ^\ 1 / 2(a-{-x)i 2. d^-^, = d[b(a + x)-i] =bd(a + x)-i 3. d(a + bx')i = i(a 4- bx') - * 2bxdx = -. — , , ,^ -.dx, ^ ' " ' (a-{-bx)* EXERCISES. Form the differentials of the following expressions: I. Va+x. Va - x\ a 2. 5- 8. II. 14. Vb-x. 3- 6. 9- 12. 15- Va - hx. 4- Va - bx\ b Vx + y. b lO. 13- Vx + y' {a + x)i. X Va-\-x. Va + bx' • {x-a)h xVa — x. Va - W (bx' - a)h y' Va - by'. Find the values of -r- in the following cases: 1 6. z« = mx -\ — . 17, M = {mx' — w)*. 40 THB DIFFERENTIAL CALCULUS. 1 8. «t = Vax -\- bx'. a + ca;' 20. u = xVa— X. a 4- X 21. u = x' Vx' + a. a — X a — X a-\- X 38. Logarithmic Functions. It is required to differentiate the function u = log X. We have Jti, - log {x + Ax) - log a; = log ^-—^- = log (l + "^j- It is shown in Algebra that we Lave log (1 + h) = M{h - W + W - etc.), M being the modulus of the system of logarithms employed. Ax Hence, puting — ^ for h, we find A a x\ 1 Ax % X + 3 x' 1 ■ etc. and. passing to the limit. du = Mdx ' X ' du d'x '' _M x' In the Naperian system ilf=: 1. In algebraic analysis, logarithms are always understood to be Naperian logarithms unless some other system is indicated. Hence we write d-\os a; 1 - - dx -^— =: — ; d-iogx = — . dx X ° X Example, , - d(axy) axdy + ai/dx dy , dx d-\os axy = --^^ — — = ^— ^ — - — = — H . axy axy y x Remabk. We may often change the form of logarithmic BIVVEUENTIATION OF EXPLICIT FUNCTIONS. 41 functions, so as to obtain their differentials in various ways. Thus, in the last example, we have log {axy) = log a + log x + log y, from which we obtain the same differential found above. The student should find the following differentials in two ways when practicable. EXERCISES. Differentiate: I. log (a -\- x). Ans. . 2. log {x — a), 3. log(:i;' + J'). 4. log (x= - S). 5. log mx. 6. log mx^. 7, log (aa;" + V). 8. log m". 9. log (x + y). 10. log {x - y). II. log a;y. 12. log (x' + ^z")- 13. log (« + &)". 14. log^. ^ x4- a ^ , a — x 115. log r-^. ID. log 7 . 1-1. y log X. 18. log (« — ar)". 39. Exponential Functions. It is required to differentiate the function u = a", a being a constant. Taking the logarithms of both members, log u = X log a. Differentiating, we have, by the last article, c?-log u = — = dx log a. 42 THE DIFFERENTIAL CALCULUS. Hence du — u log a dx^ a" log a dx; ^^ = a^loga, which is the required derivative. If a is the Naperian base, whose value is e = 2.71838 . . . . , we have log a = 1. Hence d-e" _ ^ dx ~ Hence the derivative of e' possesses the remarkable prop- erty of being identical with the function itself. EXERCISES. Differentiate: I. a^. Ans. 2a=== log a dx. 2, a"^. 3- c»+'". 4- ga + n._ 5- Jl"^ + 'iV^ 6. h-^'-y. 7- /t-*^. 8. d'ay. 9. a'bK 10. a'^b'K II. ak^b-"". 12. e* + ». 13- e'e^". 14. gOat + by^ 30. The Trigonometric Functions. The Sine. Putting h for the increment of x, we have, by Trigonometry, sin {x -\- h) — sin a; = 3 cos {x -\- ^h) sin ^h. Now, let h approach zero as its limit. Then, sin {x-\-h) — sin x becomes d sin x; h becomes dx, because it is the increment of x; cos {x -\- \h) approaches the limit cos x; sin \h approaches the limit ^U or ^dx, because when an angle approaches zero as its limit, its ratio to its sine approaches unity as its limit (Trigonometry). Hence, passing to the limit, d'sin X = cos xdx. DIFFERENTIATION OF EXPLICIT FUNCTIONS. 43 The Cosine. By Trigonometry, cos {x -\- It) — cos X = — sin {x -\- ^h) sin ih. « Hence, as in the case of the sine, d cos X = — sin x dz. Taking the derivatives, we have d sin x dx d'cos X dx — sin X. M N Fig. 8. PB = A sin r. KP — A cos X. Oeometrical Illustration. In the figure, let OX bo the unit- o radius. Then, measuring lengths in terms of this radius, wo shall have NK = sin x; MB = sin {x + li) ; ON = cos x; OM = cos (x + h); Also, supposing a straight lino from K io B, PK= - KP = KB sin PBK; PB = KB cos PBK. When B approaches K as its limit, the angle PBK ap- proaches XOK, or x, as its limit, and the line KB becomes dx. Hence, approaching the limit, we find the same equa- tions as before for d sin x and d cos x. It is evident that so long as the sine is positive, cos x di- minishes as X increases, whence <f 'cos x must have the nega- tive sign. The Tangent. Expressing the tangent in terms of the sine and cosine, we have sin X tan X = . Differentiating this fractional expression, cos xd'sin x — sin xd'cos x sin" xdx + cos' xdx d tan X = 5 ' — ^ cos X = sec" xdx, which is the required differential. COS X 44 THE DIFFERENTIAL CALCULUS. We find, by a similar process, , , - cos a; , , dx a cot X = d'—. — = — CSC xdx = -r-^- : sm X Bin X 1 ff'cos a; sin xdx rf'sec X = d cos a; cos a; cos a; = tan X sec a;rfa;; (?"cosec X — — cot a; esc xdx. EXERCISES. Differentiate: I. cos {a + 2/). z. sin (5 — y). 3. tan (c + 2). 4. sin y cos 2. 5. tan u cos v. 6. sin m tan v. 7. sin aa:. 8. cos ay. 9. tan mz. 10. sin (7* + my), ri. cos (A + my). 12. sin (A — my). 13. cos' a; • [d'cos' x = 2 cos a;<?-cos a; = — sin 2xdx]. 14. sin" a;. 15. sin' y. 16. sin' m2. sin X „ sin' a; cos' x 17. . 18. -. 19. -^-j—. cos y COB y sm y 20. Show that (?(sin' y + cos' y) = 0, and show why this result ought to come out by § 34. 21. Differentiate the two members of the identities cos {a-\- y) — cos a cos y — sin a sin y, sin {a -\- z) = cos a sin 2 -|- sin a cos 2, and show that the differentials of the two members of each equation are identical. 22. Show that d'log sin x = cot x dx; d'log cos a; = — tan x dx. 31. Circular Functions. A circular function is the in- verse of a trigonometric function, the independent variable being the sine, cosine, or other trigonometric function, and the function the angle. The notation is as follows: If y ^= sin 2, we write 2 = sin '~ " y or arc-sin y; If M = tan X, we write x = tan '~ " u or arc-tan u; etc. etc. etc. DIFFBRENTIATION OF EXPLICIT FUNCTIONS. 45 Differentiation of Circular Functions. If we have to dif- ferentiate z = sin '~ " y, we shall have y = sin z; dy ~ cos z dz = Vl — sin" a dz; • dz = ^^^ = = ^^ («) Vl - sin'' 2 +^1 - .v"' TAe Inverse Cosine. If « be the inverse cosine of y, we find, in the same way, dz=- -t=. ih) The Inverse Tangent. If we have z = tan '■" " y; then, y = tan z; dy = sec" zdz = (1 -{- tan" «)<?«; The Inverse Cotangent. We find, in a similar way, ^ 1 + 2/ y^e Inverse Secant. If we have z = sec<~'' y; {d} then, y = sec z; dy = tan z sec zdz = y Vy' — 1 <Zz; ...dz^-^^=. (e) yVf-1 The Inverse Cosecant. We find, in a similar way, «?-csc<-"V= , yVy'-l 46 TSM DIFFERENTIAL CALCULUS. EXERCISES. Differentiate with respect to x or z: 2. cos<~" (« + a). 3- sin*-" (mx + a). 4- COS*-" -. X S- tan <-»(«--). \ zJ 6. tan<-»f2+-\ 7- ''"'-L-+I)- 8. tan<-" («'). 9- sec '~ '^ 2-1 — . lO. sec<-«(.-y, II. sin^~"aa;cos^~'> — . 12. sec*-" a;' tan*" Note. — The student will sometimes find it convenient to invert the function before differentiation, as we have done in deducing the differen- tial of sin*-') X. 13. We have, by comparing the above differentials, <?(sin~^ y + COS"' y) = 0; d{ta,n~^y + cot"' y) = 0; a!(sec- ' y -\- csc~ ^ y) = 0. Show how these results follow immediately from the defini- tion of complementary functions in trigonometry, combined with the theorem of § 24 that the differential of a constant quantity is zero. 33. Logarithmic Differentiation. In the case of products and exponential functions, it will often be found that the dif- ferential is most easily derived by differentiating the logarithm of the function. The process is then called logarithmic dif- ferentiation. Example 1. Find -,- when y — x^. dx ^ We have log y = mx log x; DIFFERENTIATION OF EXPLICIT FXINOTIONS. 47 ~ =m log X dx-\- mdx; — -i = y{j^i log X -\-m) = mx^'^il -\- log x), (iX -, „ sin™ X Example 2. y = — . ■^ cos" X We have log y — m log sin x — n log cos a;; fZy _ in cos a; »t sin x^ ydx sin a; cos x ' dy sin""-' a;. , , . , . —■ = „ , , (OT cos X 4- 11 sm a;). (?a; cos" + ia;^ ' ' MISCKLLANEOUS EXERCISES IK DIFFERENTIATION. Find the derivatives of the following functions with re- spect to x: 1. y := X log X. Ans. -^ = 1 + log x. 2. 2/ = log tan X. Ans. dx dy _ 2 dx sin Hx' x. y — log cot X. Ans. ~ — : — -—. ^ ^ dx sni2x X . dy "' |/(a= - x')' dx {a' - xy dy _ nx"~ d^~"(r+F} dy _ 4 _ a;" dy _ ?ia;"~' ^- ^- (1 + a;)"' ^*" ^ " (Th^SJ^^i" •'«'" + « aa; (e'"+e "') 7. y = log {e- + e-% Ans. ^ = ^-^^^. 8. y=. log tan (J + I). Ans.^=^. X , dy e'"(l — x) — 1 9- y = ^^r ^'''- ± = (f^vr- lo „- 4/(l+^)+l/(l- ^)j,,„ f^_ 1 ^ ~ 4/(1+:^)- V(l-a;) ^a: ~ a; ^(1 - a;")" -x' 48 TBE BIFFEBENTIAL CALCULUS. 12. 2/ = tan « -. Ans. % = - '^^^ log a-a\ dx X ° 13. y^af^ Ans. j- = 7f{l + log a;). . ,^ s , dil COS (lofiT a-) 14. y = sm (log x). Ans. -j- = '^ " ■ ' . , , a; i dy 15. w = tan ' — . Ans. -— — — 7= " n - x' dx ^/i ,6. 2, = log(^)^-|-tan--.. 17. y = log/y/- - 1 — tan X A dy , , ■ \ 18. y — . Ans. -^ = — (cos x + sm x). sec a; aa; 19. y = log (log a;). ^ws, -^ = —^ . ^ ■^ s \ & / (Za; a; log a; , 1 — a;" . <fw — 2 y = sm- ' -— — 3. ^Ms. -i- = -— - — 5. 1 + a;" dx 1 -\- X , , /a cos X — b sin a; V = log y ; 7^ — -. a cos a; + 6 sm a; . dy — ah Ans. -~- = -5 3 ,a ■ ■ . «a; a" cos a; — sm' x 22. If « = — , prove the relation ^ - -I - = 0. dif 23. y =6 -«'*'. Jws. -yi = — 2a'a;y. . Jtf a;' aa; 1 — a; Vl + a;' + a: Vl + x' — X A dy 1 20. y 21. DIFFBBENTIATION OF EXPLIOIT FUNCTIONS. 49 _ 1 1 /|«o '^y ~ ^(^ + «) + n{a + x) 25. y = (a' 4- x') tan~' -. Ans. -— = 3a;tan-> — |- a. 26. y = |/^^-±|y Ans. ^y _ dx (1 — x) \/\ — x"' 11 l'^ \ A ^y 3 ^MS. -r- = sin -^.-t: -)- - — dx Vl-x' 28. y = X sin~' a;. 29. y = tan X tan~ * x. . dy J . 1 , tan x Ans. -~ = see" x tan ~'a; + - — ; — ■„-. ax 1 + a; 30. y = sin W2;(sin x)" dy 31. y = Ans. -~ = n (sin a;) "~'sin (» + l)x. (sin Ma;)"" (cos mxY' . dy ?nw (sin wa;)""' COB (wia; — wa;) (cos mx) " + * 32. y = e Ans. dx cos rx dy _ <?a; _ p-a-'x'^ e - ""*" (Sa'a; cos rx -\- r sin ra;). 33. y = log- rt + S tan ] a — h tan j '•^MS. dy dx ab 2 2**' Tt • l"*^ 34. y = a;- 35. y = sm- 4^' >.«» ^-^ _ a:^(l - lo g x) A dy ^ dx 1/1 - 3a; ■ 36. y = tan~ ' {n tan x). Ans. -^ = dx cos' a; + w' sin' a;' 60 THE BIFFMBENTIAL CALCULUS. dy 37. « = sec~*--7-j jr. Ans. -^= ,. „ ^. •" ^ y'(ffl' — z') dz y(a — z') 38. y = (a; + a) tan- ' ^|/^ - V(aa;)). ^«s. ^ = tan-'i/^. dz y a 39. y = sin- ' y(sm z). Ans. -^ — \ |/(1 + cosec z). 4°. y = tan- ' ^^-^. ^^s- ^ = 1+^.. , b + a cos z , dy — ^{a' — V) 41. V = sm- ' — ^^^ . Ans. -f- = -— '-. ^ a-\-ooo&z dz a + cos a; .a*" - 1 , dy 2nz"-'- 43. y = cos- ^ ^;p^-. Ans. ^ = - ^;,-p-^. 43. 2^ = sec '5-r— 1- ^ws. 22;" - 1" (Za; 4/(1 - a;')' 44. y=tan-^J^i^.4«.. |- = ^^ji_,-. 33. Derivatives with Eespect to the Time. — Velocities. If we have a quantity which varies with the time, so as to have a definite value at each moment, but to change its value con- tinuously from one moment to another, that quantity is, by definition, a function of the time. "We now have the defini- tion: If we have a quantity <p, expressed as a function of the . time = t, the derivative, -jr, is the velocity of increase, or rate of variation of <f> at any moment. This is properly a definition of the word velocity; but it may be assumed that the student has already so clear a con- ception of what a velocity is, that he needs only to study the identity of this conception with that of a derivative relatively to t, which he can do by the illustration of § 19. The student is recommended to draw a diagram to rep- resent the problem whenever he can do so. DIFFEBBNTIATION OF EXPLICIT FUNCTIONS. 51 EXERCISES. 1. It is found that if a body fall in a vacuum under the in- fluence of a constant force of gravity, the distances through which it falls in the first, second, third, fourth, etc., second of time are proportional to the numbers of the arithmetical progression 1, 3, 5, 7, etc., or, putting a for the fall during the first second, the total fall will be a -\- Za -\- ba -{■ la -\- etc., continued to as many terms as there are seconds. It is now required to find, by summing t terms of this progression, how far the body will fall in t seconds, and then to express its velocity in terms of t, and thus show that the velocity is proportional to the time. , Ans. (in part). The total distance fallen in t seconds will be a^. The velocity at the end of t seconds will be 2at 2. The above motion being called uniformly accelerated, prove this theorem: If a body fall from a state of rest with a uniformly accelerated velocity during any time r, and if the acceleration then ceases, and the body continue with the uni- form velocity then acquired, it will, during the next interval r, fall through double the distance it did during the first interval. Find (1) how far the body falls in r seconds; (2) its velocity at the end of that time; (3) how far, with that velocity, it would fall in another interval of r seconds; then show that (3) = 2 X (1). 3. The radius of a circle increases uniformly at the rate of m feet per second. At what rate per second will the area be increasing when the radius is equal to r feet ? Find (1) the expression for the value of the radius r at the end of t seconds, and (2) the area of the circle at that time. Differentiate this area, and then substitute for t its value in terms of r. Note that {t= — ). We shall thus have %itmr for the velocity of increase of area. 52 THE DH'FERENTIAL CALCULUS. 4. A body moves along the straight line whose equation is a; — 2y = with a uniform Telocity of n feet per second. At what rate do its abscissa and ordinate respectively increase ? I- 5. A man starts from a point h feet south of his door, and walks east at the rate of c feet per second. At what rate is he receding from his door at the end of t seconds? Ans. If we put m = his distance from his door, we shall have du _ cH dt^ u' 6, A stone is dropped from a point i feet distant in a hori- zontal line from the top of a flag-staff 9a feet high. At what rate is it receding from the top of the flag-staff (1) after it has dropped t seconds, and (3) when it reaches the ground, assuming the same law of falling as in Ex. 1? At the end of t seconds the square of the distance from the top of the flagstaff — m' = 6' -|- a'f . On reaching the ground we should have du 54a« 7. The sides of a rectangle grow unifoi-mly, both starting from zero, and the one being continually double the other. Assuming one to grow at the rate of m feet and the other 27W feet per second, how fast will the area be growing at the end of 1, 2, 10 and t seconds? How fast, when one side is 4 and the other 8 feet ? 8. The sides of an equilateral triangle increase at the rate of 2 feet per second. At what rate is the area increasing when each side is 8 feet long ? Note that the area of the triangle whose sides = « is ^ * . DIFPMRBNTIATION OF EXPLIGIT FUN0TI0N8. 53 9. A man walks round a lamp, 30 feet from it, keeping the distance with a uniform motion, making one circuit per minute. Find an expression for the rate at which his shadow travels on a wall distant 40 feet from the lamp. 10. The hypothenuse of a right triangle is of the constant length of 10 feet, but slides along the sides at pleasure. If, starting from a moment when the hypothenuse is lying on the base, the end at the right angle is gradually raised up at the uniform rate of 1 foot per second, find an expression for the rate at which the other end is sliding along the base at the end of t seconds, and explain the imaginary result when t> 10. 11. Two men start from the same point, the one going north at the rate of 3 miles an hour, the other north-east 5 miles an hour. Find the rate at which they recede from each other. 12. A body slides down a plane inclined at an angle of 30° to the horizon, at such a rate that it has slid M' feet at the end of t seconds. At what rates is it approaching the ground (1) at the end of t seconds, and (3) after haying slid 75 feet ? 13. A line revolves around the point [a, h) in the plane of a system of rectangular co-ordinate axes, making one revolu- tion per second. Express the velocity with which its intersec- tion with each axis moves along that axis,- in terms of a, the varying angle which the line makes with the axis of X. . dx 3J7r dy 2a7r Ans. — . J — dt sin" Of' dt cos" a 14. A ship sailing east 6 miles an hour sights another ship 7 miles ahead sailing south 8 miles an hour. Find the rate at which the ships will be approaching or receding from each other at the end of 30, 30, 60 and 90 minutes, and at the end of t hours. 64 THE DIFFERENTIAL CALCULUS CHAPTER V. FUNCTIONS OF SEVERAL VARIABLES AND IMPLICIT FUNCTIONS. 34. Def. A partial differential of a function of sev- eral variables is a diflEerential formed by supposing one of the variables to change while all the others remain constant. The total differential of a function is its differential when all the variables which enter into it are supposed to change. A partial derivative of a function with respect to a quantity is its derivative formed by supposing that quantity to change while all the others remain constant. Eemaek. The adjective partial may be omitted when the several variables are entirely independent. Example. Let us have the function M = a^Xy + «) + y«- (a) Differentiating it with respect to a; as if y and z were con- stant, the result will be du = 2x{y -f z)dz, (b) which is the partial differential with respect to x. Also, is the partial derivative with respect to x. In the same way, supposing y alone to vary, we shall have du = {x' + z)dy, (c) (£)-•+- PARTIAL DERIVATIVES. 55 which are the partial differential and derivative with respect to y. For the partial differential and derivative with respect to z we have du — (q? + y)dz; (d) (5) = «■+»■ Notation of Partial Derivatives. 1. A partial derivative is sometimes enclosed in parentheses, as we have done above, to distinguish it from a total derivative (to be hereafter de- fined). But in most cases no such distinctive notation is necessary. 3. In forming partial derivatives the student is recom- mended to use the form Dm instead of t— , dx because of its simplicity. It is called tJie D^ of u. The equa- tions following (i), (c) and {d) would then be written: D^u = 2x{y-\-z); DyU = a;' + «; D^ti = x' -\- y. EXERCISES. Find the derivatives of the following functions with respect to X, y and 2: 1. V =x' ~ xy -\- y". Alls. D^v = 2x — y; DyV = — x -\- 2y; D^v = 0. 2. w = x' -j- sj'y + xz. 3. w = x'y'z*. 4. u =x log y -\-y\ogx. S- u = {x -\- y -\- z)'. 6. u — \/{x + my). 7. u = (x -\-2y + 3z)i. Note. In forms like the last three, begin by taking the total differential, thus: du = i(x + 2y + 3zy*d- {x + 2y + 3z) = i(» + ^y + 3z)"* {dx + 2dy + ddz). 56 THE DIFFERENTIAL CALCULUS. Then, supposing x alone to vary, Dx^l = supposing y alone to vary. 2{x+2y+3zf D«u — 7. {x + 2y+3zy supposing z alone to vary, B^u — 2{x+2y+dzf' 8. w = {x -\- y -\- «)". 9. 10. w = cos {mx -\- y). 1 1. 12. ?; = tan (a; — y). 13. 14. V = cos' {ax -\- bz). 15. 16. u = xe^ -\- ye". 17. w = (a;' + y^ + «")». w = sin (a; + 2y + 82). V = sec (w?a; + nz). w x" + y'", 18. ?« = sin {x-\-y) cos (a;— «/). 19- u — xsm.y — y sin a;. 35. Fundamental Theoeem. The total differential of a function of several variables, all of wJiose derivatives are continuous, is equal to the sum of its partial differentials. As an example of the meaning of this theorem, take the example of the preceding article, where we have found three separate differentials of u, namely, (b), (c) and (d). The theorem asserts that when x, y and z all three vary, the re- sulting differential of u will be the sum of these partial differ- entials, namfely, du = 2x{y + z)dx -{- (a;" + z)dy -\- {x' -\- y)dz. To show the truth of the theorem, let us first consider any function of two variables, x and y. u = (p{x, y). (1) Let us now assign to x an increment /dx, while y remains unchanged, and let us call u' the new value of it, and A^u the resulting increment of u. We shall then have «' = 0(a; + Ax, y); A^u = <j){x + Ax, y) — t}){x, y). (3) TOTAL DIFFERENTIALS. 57 In the same way, if x retains its value while y receives the increment Ay, and if we call AyU the corresponding incre- ment of u, we have AyU = <p{x, y-\-Ay)- (p{x, y). (3) When Ax and Ay become infinitesimal, these increments (2) and (3) become the partial differentials with respect to x and y. Now, to get the total increment of u, we must suppose both X and y to receive their increments. That is, instead of giv- ing y in (1) its increment Ay, we must assign this increment in (3). Then for the increment of u we shall have, instead of (3), the result Ay = (t){x + Ax,y + Ay) - 4>{x + Ax, y). (4) Note that (3) and (4) differ only in this: that (3) gives the value of AyU iefore x has received its increment, while (4) gives AyU after x has received its increment, and is therefore the rigorous expression for the increment of u due to Ay. Now, what the theorem asserts is that, when the increments become infinitesimal, the ratio of Ayu' to AyU approaches unity as its limit, so that we may use (3) instead of (4). To show this, let us put ^■<--rt-(s)- Then, supposing Ay to become infinitesimal, and putting dyio for that part of the differential of u arising from dy, we shall have, from (3) and (4), dyU = cj)'{x, y)dy; (3') dyu' - <t)'{x + Ax, y)dy. (4') When Ax approaches zero as its limit, ip'{x + Ax, y) must approach the limit (p^x, y), unless there is a discontinuity in 68 TBB DlPmnENTtAL CALCULUS. the function 4>', which case is excluded by hypothesis. Thus, using (3') for (4')j we have Total differential of w = du = f-r- \dx + <j>'{x, y)dy The same reasoning may be extended to the successive cases of 3, 4, . . . n variables. The following are examples of finding some differential al- ready considered in Chap. IV., by this more general process. 1. To differentiate u = xy. du dx = «/; du — =x. dy Total differential. du- = ydx ■ + xdy. 2. X u = — = xy- ■ 1 du di=y -I . 3 du dy ■ xy ' %; du = y~ ^dx — xy ^dy = 5 — -. 3. u = ax -\- bxy -\- cxyz. ^^ = « + ^y + "y^; du, , — - = bx-\- cxz; dy du du = (a + Jy + ctjz)dx + {hx + cxz)dy + cxydz, as in § 25, Example 1. DIFFERENTIATION OF IMPLICIT FUNCTIONS. 69 EXERCISES. Write the total difEerentials of the functions given in the exercises of § 34. 36. Principles Involved in Partial Differentiation, All the processes of the present chapter are aimed at the following object: Any deriyative expression, such as du _ di' °'^^''' presupposes (1) that we hare the quantity u given, really or ideally, as an explicit function of x, and perhaps of other quantities; (3) that we are to get the result of differentiating this function according to the rules of Chap. IV., supposing all the quantities except x to be constant. Now, because it is often difficult or impossible to find u as an explicit function of x, we want rules for finding the values of D^u, which we could get if we had u given as such a func- tion of X. For example, we might be able to find the equa- tion u = <p{x) if we could only solve one or more algebraic equations. If, for any reason, we will not or cannot solve these equations, we may still find D^u whenever the equations would suffice to give u as a function of x if we only did solve them. The following articles show how this is done in all usual cases. 37. Differentiation of Implicit Functions. Let the rela- tion between y and x be given by an equation of the form 0(a;, y) = 0. (a) Eepresenting this function of x and y by 0, simply, and supposing for the moment that x and y are independent variables, so that need not be zero, we shall have, by the last section, deb = -j^dx + -r^y- ^ dx dy ^ 60 THE HmVERENTIAL CALCULUS. But, introducing the condition that equation (a) must be satisfied, tZ0 must be zero, because x and y must so vary as to keep constantly zero. We then find, from the last equation, dz d^ Dy^' ^ ' dy which is the required form in the case of an implicit function of one variable. Cor. If from an equation of the form x =f{y) we want to derive the value of D^y, we have ^{x, y)=x -f{y) = 0; <?0 _ d<p _ df(y) _ dx dx ~ ' dy '^ dy ~ dy' Hence -^ = -r-. dx dx dy Example. To find D^y from the equation 9^(*' y) = y — ax — li. Wehave g = - «; f = 1; % = a', the same result which we should get by differentiating the equivalent equation y — ax. Remabk. If -we should reduce the middle member of (1) by clearing of fractions, the result would be the negative of the correct one. This illustrates the fact that there is no relation of equality between the two differentials of each of-the quantities x, y and 0, all that we are concerned with being the limiting ratios dy -.dx; d4>: dx, and d<t> : dy, which limit- ing ratios are functions of x and y. We may, indeed, if we choose, suppose the two ds's equal and thetwo dy's equal. But in this case the two d(p'a must have opposite algebraic signs, because their sum, or the total differential of cp, is necessarily zero. Now, if we change the sign of either of the dcp's, wc shall get a correct result by a fractional reduction. DIFFERENTIATION OF IMPLICIT FUNCTIONS. 61 EXERCISES. Find the values of -—, -^ or -5- from the following equa- tions: I. y' — ax = 0. 2. y'' — yx-^-x' = 0. 3. x' + ixz + 2' = 0. 4. u{a-x)-\-u\b + x) = 0. S- log a; + log y = c. 6. log (a;+z/) + log {x-y) = c. 7. sin a; + sin y = c. 8. sin ax — sin % = e. 9. M 4" e sin ?« = X. 10. x {1 — e cos 2) = a. 38. Implicit Functions of Several Variables. The pre- ceding process may be extended to the case of an implicit function of any number of variables in a way which the following example will make clear. Let u be expressed as a function of x, y and z by the equation u' + xu' + {x' + y')u + x' + y' + z' = 0. Since this expression is constantly zero, its total differential is zero. Forming this total differential, we have {3u' + 2xu + x' + y')du + («' + 2ux + 3x')dx + {2uy + 3y')dy + 3z'dz = 0. By § 34 we obtain the derivative of u with respect to x by supposing all the other variables constant; that is, by putting dy = 0, dz = 0, and so with y and z. Hence dtt _ u' -\- 2ux -4- Sa;' -^ = Dm = dx " 3u' + 2ux + a;" + y" du _ _ 2m/ + 3y^ --_V^u- - 3jj= _^ 2ux + a;' -f y" du _ 3z^ -rr- = Dm dz ' 3u' + 2ux + a;" + y'' 62 THE DIFFERENTIAL CALCULUS. EXERCISES. Find the derivatiyes of u, v or r with respect to x, y and e from the following equations: 1. xu' + ifu' + z'u — x'yz. 2. a cos {x — ti) -\-b sin {x -\- u) = y. 4. r- a! + » I ij-x — v ^ jje_ 6. e"" cos a; + e"" cos y = e". 8. V° + 'HVX cos -f k" r= 5", 3. u" + zt" = ?«'. 5. V log a; + 2 log V = y. 7. ?t" — Swa; cos z -f- a;' = a', 39. Case of Implicit Functions expressed by Simvlta- neotis Equations. If we have two equations between more than two variables, such as F^u, V, X, y, etc.) = 0, F^{u, v, x, y, etc.) = 0, then, if values of all but two of these variables are given, we may, by algebraic methods, determine the values of the two which remain. We may therefore regard these two as func- tions of the others, the partial derivatives of which admit of being found. In general, suppose that we have n independent variables, a;,, x^. . . ar„, and ?« other quantities, u^, u^ . . . Un, connected with the former by m equations of the form F^u^, M, . . . M„, a;,, a;, . . . a;„) i^,(M., M, . a;„) = («) i^m(w„ M, . , . Mm, «„ a;, . . . a;„) = 0. By solving these m equations (were we able to do so) we should obtain the m u'& in terms of the n a;'s in the form Ml = 0,(a;., a;, . . . a;„); Mr.= 0m(a;„a:, . . . a;„); _ (6) DIFFERENTIATION OF IMPLICIT FTJNGTIONB. 63 and by differentiating these equations (5) we should find the mn Talues of the deriTatives -j-'; -=-'; . , . -r^; etc. 'Now, the problem is to find these same derivatiyes from (a) without solring (a). The method of doing this is to form the complete differen- tial of each of the given equations (a), and then to solre the equations thus obtained with respect to clu^, du^, etc. The results of the differentiation may, by transposition, be written in the form dF, , , dF,^ , , dF, ^ dF^ , , ^r^ du, 4- -^ du, + . . . + t— du^ = — ^ dx, — etc. : du^ ' du, ' du„^ dx^ ' dF, , , dF,^ , , dF,^ Fd, , , dF^ , , dF^ _, , , dF^ -. dF^ , , _^^,.+__,?^, + . . . +_^«„:=: __^.,_ etc. By solving these m equations for the m. unknown quantities du^, du, . . . dUm, we shall have results of the form du^ = M^dx^ + M,dx, + • ■ • + ^ndx„; du, =? W^dx^ + JV,dx, + . . . + NJLx^; etc. etc. etc. etc.; where M^, iV„ etc., represent the functions of u^. . . u^, x^ . . . x„, which are formed in solving the equations. We then have for the partial derivatives ^i_jf. ^^-M- etc Example. Prom the equations rcosd = x,) ^^,j r sin 6* = y, ) it is required to find the derivatives of r and 6 with respect to X and y. 64 THM DIFFERENTIAL CALCULUS. By differentiation we obtain cos ddr — r sin 6d6 = dx; sin 6dr + r cos 6d6 = dy. Multiplying the first equation by cos 6 and the second by sin 6, and adding, we eliminate dO. Multiplying the first by — sin 6 and the second by cos 6, and adding, we eliminate dr. The resulting equations are dr = cos ddx + sin ddy; rdd = cos ddy — sin ddx. Hence, as in the last section, (I) = ^°^ ^' (S) = '^ ^' ldff\ _ _ sva_e_ lde\ _ cosg Wk/ ~ r ' \f^y/ ~ r EXERCISES. 1. From the equations >• sin ^ = a; — y, r cos 6 = x-\- y, find the deriyatives of r and ^ with respect to x and y. 2. From the equations • Me" = r cos ^, ?(e~"= r sin ^, find the derivatives of u and v with respect to r and 6. Ans. [-^) = Ue" sin (? + e - " cos (9) ; ^(e-''sin (9 + 6" cos (9). (-) = \de} FUNCTIONS OV FUN0TI0N8. 65 3. From the equations u" -\- ru = x' -\- y', m" — ru = xy, And the derivatives of r and u with respect to x and y. 4. From the equations 5. From a;" + «/' + «' - '^xyz = 0, m' - 3«;« cos 6 -^r z'' - a', x-\-y -\- z = a, id' -\- 2uz cos 6 + z" — b', „ ■, dz J dz „ . du du dw dw find J— and -j-. find -3-; -=^; -r~; -ttt. dx dy dz' dd ' dz d6 40. Functmis of Functions. Let us have an equation of the form u-f{<P, t, 6, etc.); {a) where <p, ip, d, etc., are all functions of x, admitting of being expressed in the form <P^fA^); i^=.a^); 6=f,{x); etc. {b) If any definite value be assigned to x, the values of cp, f, 6, etc., will be determined by [b). By substituting these val- ues in (rt), u will also be determined. Hence the equations («) and {b) determine u as a function of x. By substituting in (a) for 0, tp, B, etc., their algebraic expressions /,(a;), fi{x), etc., we shall have u as an explicit function of x, and can hence find its derivative with respect to X. But what we want to do is to find an expression for this derivative without making this substitution. By differentiating (a) we have ^„ = ^^0_^grf^ + ^^^ + etc. By differentiating (^), d(t> = -^dx\ dtp — -^dx; d6 = -^f-dx; etc. dx dx dx 66 THE DIFFEBENTIAL CALCULUS. By substituting these values in tlie last equation and divid- ing by dx, we have du _ du d(p die dij) du ^^ _, j. /-,\ dx d<p dx d^ dx dQ dx ' ^ ' The significance of this equation is this: a change in z changes u in as many ways as there are functions (p, ip, 6, etc. -5--r -^dx is the change in u through ^; -jj -T-dx is the change in u through ip; etc. etc. The total differential is the sum of all these separate infinitesimal changes, and the derivative is the quotient of this total differential- by dx. EXERCISES. 1. Find -5— from the equations u ^ a sin {inv -f- w) + 5 sin {mv — w); «; =: c -f- ii^'> w = c ~ nx. We find -=- = am cos (mv -\-w)-{-bm cos (mv — w); -,— = n; dv ' dx du , , ^ I , s dw -— = a cos (mv + «c) — J cos (mv — w); —- =: — n: dw " dx Whence, by the general formula, -— = an{m — 1) cos (jnv -\-w)-\- bn^m -\- 1) cos {rrm — w). 2. Find ^— from dx w = e* + e*; •3. Find -Y- from v' + v<p + ip'' = a; = OT(a + y); f = ny. FUNCTIONS OF FUNCTIONS. 67 4. Find -^- from r cos X — r sm x = a — y, X = inz-\-h; y = 00s nz. S. Find -=- from ^ dz r' + ar' + 2/V + 0= = 0; a;" + az = 0; y' -{■ az' = 0; = mz. 41. The foregoing theory applies equally to the case in which the function is one of two or more variables, some of which are functions of the others. For example, if u = (t>{x, z), {a) then, whatever be the relation between x and z, we shall always have, for the complete differential of u, au=[^)dx + (^)dz. Suppose that x is itself a function of z. We then have , dx ^ dx = ^ dz. dz By substitution in the first equation we have du — Wa; Idu Idz + Vdz dz; du _ Idu \ dx Idu \ . . ''"dz~\dxJ dz'^Vdz)' ^*^ The two values of -=- which enter into this equation are different quantities. A change in z produces a change in u in two ways: first, directly, through the change in z as it appears in (j); second, indirectly, by changing the values of X in (a). The first change depends upon (-j-j in the second 68 THE DIFFERENTIAL CALCULUS. member of (5); the second uponl-^j --^; while the first mem- ber of {b) expresses the total change. It is in distinguishing the two values of a derivative thus obtained that the terms 2}(t'>'Hal derivative and total derivative become necessary. If we have a function of the form w =f{^> y,w . . . z), in which any or all of the quantities x, y, w, etc., may be functions of z, then the partial derivative of u with respect to z means the derivative when we take no account of the variations of x, y, w, etc.; and the total derivative, with respect to z, is the derivative when all these variations are taken into account. In such cases the partial derivative has to be distinguished by being enclosed in parentheses (§ 34). This is why the last equation is written du _ fdu\ ldu\ dx dz ~ \dzJ \dx J dz' 43. Extension of the Principle. The principle involved in the preceding discussion may be extended to the case of any number of independent variables and any number of functions. If we have r = 4>(u, V, w . . . X, y, z . . .), in which x, y, z, etc., are the independent variables, while u, V, w, etc., are functions of these variables, we shall have *=(!?)*' + ©'"+■■• + ©*+*■ Then, since u, v, lu, etc., are functions of x, y, z, etc., we have du = -j—dx + T— </t/ + etc. ; dx dy dv = -^dx + -^dy 4- etc. dx dy •' FUNCTIONS OF FUNCTIONS. 69 By substituting these values in the preceding equation we find* dx ~~ {\dxj \duj dx \dv J dx • r(d<p\ /d(p\ du fd<p\ dv "*" LW j "^ [duj dy '^\dvl dy'^ " • + e, writing r for cp, its equivalent, cZr /«?r\ 1 dr\du ldr\dv dx ~ [dx] \dujdx Kdvjdx "' t etc. etc. etc. etc. EXERCISES. The independent variables r and d being connected with x and 2/ by the equations a; = r cos d, y =^ r sin 6, it is required to find the derivatives of the following functions of X, y, r and d with respect to r and 0. We call each of the functions u. I. u — r" + 2xy cos 36*. Here we have — - = 2w COS 28; -— = 2x COS 38; dx . dy -,— = cos 8; -f- = sme; dr dr dx . . dy . — „- = — r sm 8 = — «; -^/r = r cos 8 = a;. flO BO * Here, when we use the symhol (p instead of r, there is really no need of enclosing the partial derivatives in parentheses. We have done It only for the convenience of the student. 70 THE DIFFERENTIAL CALCULUS. _ du ldv\ , du dm , du dv Hence -r- = (3-I + -j- 3- + -j- -r- dr \drf dx dr dy dir = 2r4-33rcose coa29 + 2a:sm9 cos 29 = 2r(l + cos 29 sin 29) = r{2 + sin 49); and, in the same way, ~ = 2r^ cos 49. We might have got the same result, and that more simply, hy sub- stituting for X and y in the given equation their values in terms of r and 6. But in the case of implicit functions this substitution cannot be made; it is therefore necessary to be familiar with the above method. 2. u^\ + '' J cos 2ft r a 3- '' = x' + y' r''- 4. u^r' - {x- yy. S- 1 ~ xsmllO -\- y cos 2^* 6. 1 1 U = -^ — : ^^. X COS 20 y sin 2^* 7. u = r' -\-x' — y\ Let V and w be given as implicit functions of p and d by the equations v' + w= = 2p sin ft f ^"' It is required to find the total derivatives of the following functions with respect to p and 6 respectively: 8. ti = v' -\- w' — p'. g. u = v' — 2vw cos + w'. 10. V, — — . II. w = (y + w) sm ft 12. u ^= {v — to) cos ft 13. M = iw" — f " -)- 2(?<; "l" v)p cos ft PARTIAL DERIYATIVES. 71 From the pair of equations (a) we find do _ « dw _ w ^ ~2p'' dp~ %p' = i« cot 9; -^- z^w cot which values are to be substituted in the symbolic partial derivatives of il. 43. Reniarhs on the Nomenclature of Partial Derivatives. There is much diversity among mathematicians in the no- menclature pertaining to this subject. Thus, the term " par- tial derivative" is sometimes extended to all cases of a deriva- tive of a function of several variables, with respect to any one of those variables, though there is then nothing to distinguish it from a total derivative. Again, Jacobi and other German writers put the total deri- vatives in parentheses and omit the latter from the partial ones, thus reversing the above notation. If we have to express the derivative of <p{x, y, z, etc.) with respect to z, the English writers commonly use the symbol -T- in order to avoid writing a cumbrous fraction. We thus have such forms as dxW '^ V '^ c'J' each of which means the derivative of the expression in paren- theses with respect to x, and which the student can use at pleasure. 44. Dependence of the Derivative upoti the Form of the Function. Let x and y be two variables entirely independent of each other, and u = (p{x, y) {a) a function of these variables. Without making any change in u or x, let us introduce, instead of y, another independent 72 THE DIFFERENTIAL OALGULUS. variable, z, supposed to be a function of x and y. Then, after making the substitution, -we shall have a result of the form u = F{x, z). (b) Now, it is to be noted that although both zi and x have the same meaning in {b) as in («), the value of -^ will be differ- ent in the two cases. The reason is that in («) y is supposed constant when we diiferentiate with respect to x, while in (J) it is z which is supposed constant. Analytic Illustration. Let us have u = ax' -\- by'. This gives du - T— = %ax. ax {0) Let us now substitute for y another quantity, z, determined by the equation z = y -\-x or y = z — X. We then have u = ax" -\- b{z — xY; — - = 2ax -\- 2b(x — z); which is different from (c). Our general conclusion is: The partial derivative of one variable with respect to another depends not only upon the re- lation of those two variables, but upon, their relations to the variables which we sup- pose constant in differen- tiating. Geometrical Illustra- tion. Let r and be the polar co-ordinates of a point P, and x and y its rectangular co-ordinates, q Then X •R y 1 3fr—y Q \ ^y •'^S y^ y' / ,-' J^ #' /V^' / / /> y^' y>^ o hX x: Fio. 9. r cos 6; y = >• sin d; r' = a' + f. {d) PARTIAL DEBIVATIVEB. 73 Eegarding r as a function of x and y, we haye But we may equally express r as a function of x and 6, thus: r = a; sec 6. (/) We then have -^ = sec ft (gr) Referring to the figure, it will be seen that we derive (e) from {d) by supposing x to vary while y remains constant; that is, by giving the point P an infinitesimal motion along the line PQ || to OX. In this case it is plain that the incre- ment of r (SQ) is less than that of x. But in deriving (g) from (/) we suppose x to vary while 6 remains constant. This carries the point P along the straight line OPR; and now it is evident that the resulting increment of r {PR) is greater than that of x. 74 THE DIFFERENTIAL CALGULUB. CHAPTER VI. DERIVATIVES OF HIGHER ORDERS. 45. If we have given a function of x, y = <>(x), we may, by difEerentiation, find a value of ■—. This value will, in general, be another function of x, which we may call 4>'{x). Thus we shall have Ifow, this function 0' may itself be differentiated. If we call its derived function (f>", we shall have 4- ax dx dx = 0"(^). («) Let us examine the geo- metrical meaning of this equation, by plotting the curve representing the origi- nal equation y = <p (x). Let X, x' and x" be three equidistant values of the ab-~ scissa, so that the increments x' — X and x" — x' = Ax are equal. Let P, Q and R be the corresponding points of the curve, be the three corresponding values of y. B M AX Xo X2 Fia. 10. Let y, y' and y'' DEBIVA TIVES OF EiaHEB 0BBEB8. 75 Then we may put Ay = y' -y = MQ, A'y = y" -y' = NR, as the two corresponding increments of y. It is evident that these increments will not, in general, be equal; in fact, that they can be equal only when the three points of the curve are in the same straight line. If D is the point in which the line PQ meets the ordinate of R, then DR will be the difference between the two values of Ay, so that we shall have BR = A'y — Ay ■= increment of Ay. Hence, again using the sign A to mark an increment, we shall have BR = A Ay = A'y, (b) in which the exponent does not indicate a square, but merely the repetition of the symbol A. Thboeem I. When Ax becomes infinitesimal, A'y becomes an infinitesimal of the second order. For, if B be the point in which PQ produced cuts the ordinate X,i2, we shall have, in the triangle QRB, nB=QB'^^ = A'y. (b) sm QRB " ^ ' When Ax becomes an infinitesimal of the first order, so do both QB and the angle RQD, but the angle QRB will remain finite, because it will approach the angle QBN as its limit. Hence the expression will contain as a factor the product of two infinitesimals of the first order, and so will be an infini- tesimal of the second order. Since both the quantities QB and RQB depend upon Ax, we conclude that the ratio A^ Ax' may remain finite when Ax becomes infinitesimal. In fact. 76 THE DIFFERENTIAL 0ALCULU8. from the way we have formed these quantities, we have hm. ^ = lua.-^ = -^ = 0'». Hence — Theorem II. If we take two equal consecutive infinitesimal incremsnts, = dx, of the independent variable, then — 1. The difference between the corresponding infinitesimal increments of the function divided by dx' will approach a certain limit, - 2. This limit is the derivative of the derivative of the function. Def. The derivative of the derivative is called the second derivative. The derivative of the second derivative is called the third derivative, and so on indefinitely. Notation. The successive derivatives of y with respect to X are written dy . ^. ^. . . dx' dx" dx" ®°-' or D^y; D^y; D^^'y; etc. 46. Derivatives of any Order. The results we have reached in the last article may he expressed thus: If we have an equation y = <P{x), the first derivative is given by the equation Then, by differentiating this equation, we have, by the last theorem, dx d^y ,,,, . DEBIVATIVES OF EIOBEE ORDERS. 11 Again, taking the derivative, we have and we may continue the process indefinitely. EXERCISES AND EXAMPLES. 1. To find the successive derivatives of ax'. and all the higher derivatives will vanish. Form the derivatives to the third, fourth or ?tth order of — 2. ax\ 3. hx~^. 4. {a-\-xY. 5. {a — xy. 6. (a-j-a;)~l 7. (a — x)~^ 8. [a" + xy. 9. -Zd'x' + x\ 10. a-\-hx -\- ex' -\- hx' + kx\ 11. 1 + a; + a" + a;' + »* + a;' + . . . + a;». 12. 1 - a; + a;= - a;' + a;' - a;' + . . . H- (- l)"a;"- 13 n 13. a?. 14. a;2. 15. (rt -(- a;)" 16. (a + a;)^. 17. If «/ = e', find Z*/;;/ = a''(log a)". 18. From y = m(f, find the «th derivative. 19. From y = me*'' show that D^y = A"«/. Find the first three derivatives of the expressions: 20. 2^ 21. aof. 22. x'^"- 23. log X. 24. log (a + x). 25. »7i log X. 26. log (ffl — a;). 27. log (a + "*-'^)' ^8. log (a — ma;). 29. Show that if y — sin x, then -j^ = — y. d'y _ _ tf + ^y _ ^ W ^^^'' (?«" + " ~ <?a;»' 78 THE DIFFERENTIAL CALCULUS. 30. Show that the same equations hold true H y = cos x or if y = a cos a; + J sin x. 31. Find the law of formation of the successive derivatives of sin mx and cos mx. Especially, the {n + 4)th derivative = wth der. X what? (« + 2)th derivative = wth der. x what? 32. Find the «th derivative of e^". 33. Find three derivatives of e"" sin nz. 34. If M = y", show that ^ = (1 + log y)-^ + ^. 35. Find two derivatives of m = tan z. 3fr. Find two derivatives of m = cos" z. 37. Find two derivatives of m = sec' z. 38. Find two derivatives of m = cos' z — sin' z. 39. Find two derivatives of « = cos 22. 40. Find two derivatives of u = e'"'. 41. Find two derivatives otu = sin'~"a;. 47. Special Forms of Derivatives of Circular and Ex- ponential Functions. Because cos x = sin {x -\- ijt) and — sin a; = cos (z -f ^tt), the derivatives of sin x and cos x may be written in the form Bx sin X = sin {x -f- ^tt) and Dx cos x = cos (x -j- ^n). Hence, the sine and cosine are such functions that their derivatives are formed ly increasing their argument iy ^rr. Differentiating by this rule n times in succession, we have _ . . d" sinx . f , n \ Bx- cos X = -^— = cos [x + ^^j; results which can be reduced to the forms found in Exercises 29 and 30 preceding. DMBIVATIVE8 OF HIGHEB 0BDEB8. 79 48. Successive Derivatives of an Implicit Function. If the relation between y, the function, and x, the independent variable, is given in the implicit form f{x, y) = 0, then, putting u for this expression, we have found the first derivative to be du dy _ dx . , dx ~ du' ^ ' dy The values of both the numerator and denominator of the second member of this equation will be functions of x and y, which we may call X^ and Y^. We therefore write ^y=-^. (V) dx y; ^"^ Differentiating this with respect to x, we shall have d-'y _ ' dx '^ ' dx dx' ~ y; ^"^ X^ and Y^ being functions of both x and y, we have (§ 41) dx \dx I \ dy Idx' dT\ ^ fdY,\ fdYXdy dx \ dx I \dy Idx' Substituting in these equations the values of -^ from (J), and then substituting the results in (c), we shall have the re- quired second derivative. The process may then be repeated indefinitely, and thus the derivatives of any orders be found. Example. Find the successive derivatives of y with re- spect to X from the equation x' — xy -\- y^ = u = 0, 80 THE DIFFERENTIAL 0ALCULU8. We have £ = Zx-y; ^-=-x + %y; dy 2a; -y . dx X- 2y' ^" ' which is a special case of (a) and (b), and where X^ =: 2x — y and F^ =: — a; + 2y. Differentiating the equation {a'), we haye d'y_('= ^y) dx ^^"^ y'~ dx dx' (x - 2yy (x - 2yy Substituting the value of ~ from (a'), we have dy ^ (x - 2y) (- 3.y) + 3a:(2a; - y) dx' {x — 'ZyY _ 6 (a:' — xy -f y') _ 6m (a: - %yY ~ {x - 2yy' EXERCISES. Find by the above method the first two or three derivatives of V with respect to x, y or z, from the following equations: , \ J d'v 2(a -\- v) 1. zv = a(v — z). Ans. -t-=- = -r — ■ — ri. ' dz' (a — zf 2. v'y -(- vy' = a. 3- v' -\- vz -\- y' = b. 4. v{a — xy + v'{b -\-x) = c. 5. log {v + z)+ log (y ~z) = c. 6. sin inv — sin ny = ^. 7. t)(l — a cos «) = h. 8. If M — e sin w = (/, show that d'u 1 — e dedg (1 — e cos «)** DERIVATIVES OF HIGHER ORDERS. 81 49. Leibnitz's Theorem. To find the successive deriva- tives of a product in terms of the successive derivatives of its factors. Let iiv =j9 be the product of two functions of x. By suc- cessive differentiation we find dp _ dv du_ dx~ dx dx' d^p _ d'v du dv d'u dx' dx" dx dx dx" ' d'p _ d'v du d'v d'u dv d'u dx" ~ dx' dx dx" dx" dx dx' So far, the coefficients in the second member are those in the development of the powers of a binomial. To prove that this is true for the successive derivatives of every order, we note that each coefficient in any one equation is the sum of the corresponding coefficient plus the one to the left of it in the equation preceding. Now, let us have for any value of n d'^v d^v , du d^-^v , ^ , , the successive coefficients being 1; n; (l); (^^]; etc. (Comp. § 6.) Then, in the derivative of next higher order the coefficients will be '■■'+'■■ ©+» " m^ and, in general. (?)+fe) »' m- I'^ + ^r. That is, J „^i is formed from (a) by writing n -\-l for n. Hence, if the rule is true for n, it is also true for n-\- 1. But it is true for w ?= Sj . • , for w = 4, etc, indefinitely. 82 TEE DIFFERISNTIAL CALCULUS. 50. Successive Derivatives with respect to Several Equi- crescent Variables. Studying the process of § 45, it will be seen that we supposed the successiye increments of the inde- pendent variable to be equal to each other, and to remain equal as they became infinitesimal, while the increments of the functions were taken as variable. This supposition has been carried all through the subsequent articles. Def. A variable whose successive increments are supposed equal is called an equicrescent variable. We are now to consider the case of a function of several equicrescent variables. If we have a function of two variarbles, u = (t>{%, y), the derivative of this function with respect to x will, in general, be a ftinction of x and y. Let us write -^ = Mx,y). Now, we may differentiate this equation with respect to y -ivith a result of the form du -^ = (p^,^{x, y). Using a notation similar to that already adopted, we rep- resent the first member of this equation in the form dxdy' In the D-notation this is written In either notation it is called "the second derivative of u with respect to x and y." As an example: If we differentiate the function u = y'' sin (rnx — ny) (a\ BEBIVATIVEa OF HIGHER ORDEBB. 83 with respect to x, and then differentiate the result with respect to y, we have DxU = -J— = i^y cos (mx — ny) ; d'u D'x,yU — -J = '^my cos (mx — ny) -\- mny" sin {vix — ny). 51. We now have the following fundametftal theorem: d'^u __ d'li, _ dxdy dydx' or, in words. The second derivative of a function with respect to two equicrescent variables is the same whether we differentiate in one order or the other. Let u = <p{x, y) be the given function. Assigning to x the increment Ax, we have ^ ^ ^{x + Jx, y) - (p{x, y) Ax Ax ' ^ ' All In this equation assign to y the increment Ay, and call A—r- ^x Att the corresponding increment of --r-. Then the equation will give ^ + ^^= 0C-g + Ax,y + Ay) - cjy{x, y + A y) Ax Ax Ax ' ' Subtracting (1) and dividing the difference by Ay, we have ^^ _ 0(g +Jx,y-\- Ay) - (t>{a:, y J ^ Ay) - ct>{x + Ax. y ) + <;6(a, y ) Ay Ax Ay The second member of this equation is symmetrical with re- spect to X and y, and so remains unchanged when we inter- change these symbols. Hence we have ,Au .Au Ax _ Ay Ay ~ Ax 84 THE DIFPBBENTIAL CALCULUS. for all values of Ax and Ay, and therefore for infinitesimal values of those Lucrements. Thus ^chi , du d-^ d--^r- dx _ ay dy ~ dz ' or D\,yU = D\,^u, as was to be proved. As an example, let us find the second derivative of (a) in the reverse order. We have -^ = %y sin (mx — ny) — ny" cos {mx — ny); ay d^u -— -- = %ny cos {rnx — ny) -\- mny^ sin {vix — ny); the same value as before. Corollary. 7" he result of talcing any number of succes- sive derivatives of a function of any number of variables is independent of the order in ivhich ive perform the differcntia- iions. For, by repeated interchanges of tw^o successive differentia- tions, we can change the whole set of differentiations from one order to any other order. If we have I differentiations with respect to x, m with re- spect to y, n with respect to z, etc., and use the Z>-notation, we express the result in the form Here the symbol D^ means DyBy, etc., m times. In the usual notation the same operation is expressed in the form cF + ™ + " + ---0 dofdy'^dz" . . .' The corollary asserts that, using the Z>-notation, we may permute at pleasure the symbols DJ, B^, D^, etc., without changing the result of the differentiations. DMHIVATIVES OP EI&EER ORDMBS. 85 EXERCISES. Verify the theorem DJ)yU = DyD^u in the following cases: I. u = X sin y -\- y sm x. 2. u = x". 3. u = xlogy. 4. u = a sin {x -\- y) — b sin {x — y). Differentiate each of the following functions once with re- spect to z, twice with respect to y, and three times with re- spect to X, in two different orders, and compare the results. 7. xsva. y -\- 2/ sin z -\-z sin x. 8. sin {Ix -\- my + nz). 1 9. If u — 'V ^{x' + f-\- z') , ,, , d'u , d''u , d'u show that -_+_, + _, = 0. 53. Notation for Powers of a Differential or Derivative. Such an expression as du' may he ambiguous unless defined. It may mean either Differential of square of u; i.e., diu'); or Square of differential of 11; i.e., {du)'. To avoid ambiguity, the expression as it stands is alwayn supposed to have the latter meaning. To express the differ- ential of the square of u we may write either d'u' or (?(«"), of which the first form is the easier to use. The square of the derivative -p- may be written either Idu y du* \dx I do?' 86 THE BIFFEItmNTIAL CALCULUS. CHAPTER VII. SPECIAL CASES OF SUCCESSIVE DERIVATIVES. 63. Successive Derivatives of a Power of a DerivativBc Let us have to differentiate the derivative ■with respect to x. In such operations the i^-notation will he found most con- venient. Applying the rule for differentiating a square, the result is j/duV du \dx) _ du dx _ du ^ dx dx dx ~ dx dx" or, in the i)-notation, D^iD^u)' = 2D^uDM In the same way, we find d-{I)^uY (duy-^d'u ,„ ^„ .„, SPECIAL CASES OF SUCCESSIVE DERIVATIVES. 87 EXERCISES. Write the derivatives with respect to x of the following ex- pressions, y being independent of x when it is written as an equicrescent variable: ■• (I)" '■ (I)" - (I)' ^- ^©' »• '©■■ '■ 4)" 7- \w ' ^- wj • ^' \^ ■ ^°' d«^' ''■ \dxl dy ' \dxl \dyl ' (dyVlduV (duV/d'uy IdhiVld'uV '3- liJ \dz) ' ''• \di) \d^} • '5- [ay) W-j ' fd'yVfd'yY dudv_dudv fd^uV 54. Derivatives of Functions of Functions. Let us have, as in § 40, M=/-W, (1) where ^ is a given function of x. It is required to find the successive derivatives of u vrith respect to x. We may evi- dently reach this result by substituting in (1) for f its ex- pression in terms of x, and then differentiating the result by methods already found. But what we now wish to do is to find expressions for the successive derivatives without making this substitution. To do this, assign to x the infinitesimal increment dx. The re- sulting infinitesimal increment in ip will be df = ^dx. 88 TaS DIFfERBNTlAL CALOVHTS. This, again, will give u the increment du = g#, or, by substituting for di/^ its value, and passing to the de- rivative, du _ du dip ,„, dz dip dx' This is a particular case of the result already obtained in §40. The second member of (2) is a product of two factors. The first of these factors is formed by differentiating a func- tion of ip with respect to if); and is therefore another (derived) function of ip; while the second is, for the same reason, a function of x. Differentiating (2) with respect to x by the rule for a prod- uct, we have* ^du d'u _dip dip du d'lp . . dx' ~ dx dx dip dx'' ^ ' N"ow, because -rr is a function of ij), its derivative with re- dip spect to X is to be obtained in the same way as that of u. If we put, for the moment, «' = !=/•»). we have, as in (3), du' _ du' dip _ d^u dip ^ dx ^ dip dx ~ dip' dx ' * The student should note that the expression —: — cannot be put in the cPu form — r-;-, because the latter form presupposes that ib and x are two in- dijidx dependent variables, which is here not the case. In fact, v, does not con- tain X except in ip. SPECIAL OASES OF SUCCESSIVE DEBI7ATIVE8. 89 and hence, by substitution in (3), d'u _ d'uldipV du d'lp ,... d^' ~ dip'W) "^ # d^"'' ^ ' which is the required expression for the second derivative. From this we may form the third and higher derivatives by again applying the general rule embodied in (2), namely : Ifipisa function of x, we find the derivative of any func- tion, u, ofiphy differentiating u with respect to ^, and mul- tiplying the resulting derivative by -r-. From the equation (4) we have d^ _ /dtpV df^ ^d^ dtpd'i/} dx' ~ \dx I dx dip' dx dx' ^ du d'lp dtp du d'lp dx' dx dip dx' ' By the rule just given, we have T^d'u dip ' _ d\i dip ^ dx ~ dip' dx' ^du dtp _ d'u dip dx ~ dip' dx ' Hence, by substitution and aggregation of like terms, ^ _ d'u fdipy ^u d'lp dip du d'lp dx' ~ di/Adx I "^ dip' dx' dx + dip dx' ' ^^' Eepeating the process, we shall find d'u _ d^/dipy d'li^ d'lp fdipy dx* ~ dip\dxj + dip' dx' \dxl ^u r^ # /^vn , du d'lp .„. ■*" dip' [sdx' dx^''\dx'i A'^dfd^- y^> 90 TEE DIFFERENTIAL OALOULUa. Example. Let ua take the case of M — sin ip, ip being any function whatever of a;. We may then form the successive derivatives as follows: du du dtb ,dib J- = jT :r- = cos ib^; dx dtp dx ^ dx d'u . ,fdip\\ ,d'tp d*u ,ldip\' „ . .dipd'ip jdipY „ . .dfd'i/} , , d'f EXERCISES. Putting = a function of x, find the first three derivatives of the following functions of (p with respect to x: I. u = cos (p. 2. M = <p' 3. M = 0". 4. M = 0". 5. u = log 0. 6. M = e* . ■J. u = sin 30, 8. M = cos 30. 55. Change of the Equicrescent Variable. Let the relation between y and a; be expressed in the form X = 4>{y), (1) and let it be required to find the successive derivatives of y with respect to x, regarding the latter as the equicrescent. We may do this by solving (1) with respect to y, and then differentiating with respect to x in the usual way. But the method of the last article will enable us to express the required successive derivatives of y with respect to x in terms of those of x with respect to y, which we can obtain SPECIAL OASES OF SU00ES8IYE DERIVATIVES. 91 from (1). By differentiating (1) as often as we please, we have results of the form D,^x = ct>"y =^"; [ (2) B^'x=<t>"'y = x"'.) etc. etc. x\ x", x"'f etc., thus representing functions of y, I'rom § 37, Cor., we have ^ = -J- = - . (3) dx DyX x'' ^ ' To obtain the second derivative, we hate to differentiate x', a function of y, with respect to x (§ 54). Thus 3^y 1^ dx' dy_ dx* ~ x"' dy dx' From (3), ^' = f? = x". '' dy dy •^ Prom this equation and (3) we have d^ ^ - _ E" - _ .^ ,A^ dx'~ x"~ fdx_y' W Differentiating again, we find ^ ^ (3x^ dx^__l dx^'\ dy dx'~\x'* dy x" dy) dx of^A' <^a; d'z \dyV ~dvdi? _ 3x"' - x'x'" _ \dyV dy dy ~ x" ~ ' /dxV [dyj The above process may be carried on to any extent. But many students will appreciate the following more elegant method of obtaining the required derivatives. Imagine that we have solved the equation (1) so as to obtain a result in the form y = F{x). (5) 92 THE DIFFERENTIAL CALCULUS. If in this equation we substitute for x its value (1), we shall have a result in the form y = F{cPy), (6) which, of course, will really be an identity. But we may still differentiate (5) with respect to y, regard- ing a; as a function of y given by (1), by the method of §§ 40 and 54. Thus we shall have <?'y _ d'yldxV d'y d'x dx dy d'x dy' ~ W'Xdy I dx' dy' dy dx dy'' etc. etc. etc. etc. But from the identity (6) y = y, which is obtained from (5), we have ^-1- ^-0- ^l-Q- etc dy - ^' df - "' dy' ~ "' ®*°- Therefore, substituting for the derivatives of x with respect toy the expressions x', x", etc., in (4), we have the equations x'^-1- "" dx-^' ^ dx'^'^ dx- "' dx' dx dx ' a;" %! + ^x"x" S + (4a;'a;"' + 3a;'") ^ + x'^% = 0. dx* dx ' dx' dx Solving these equations successively, we shall find the values of -r-, -t4j etc., already obtained. 56. Case of Two Variables Connected by a Third. The case is stiU to be considered in which the relation between x 8PWIAL OASES OF 8U00E88IVE DERIVATIVES. 93 and y is expressed in the form y = 4>M; a; = 0,(m). (1) From these equations it is required to find the successive deriratiye of y with respect to x. The first derivative is given by the equation dy_ dy _ du _ J)„y dx dx B^x' du From the manner in which the second member of this equa- tion is formed, it is an explicit function of u alone. Hence (§ 54) we obtain its derivative with respect to x by taking its derivative with respect to u, and multiplying by -j—. Thus dx d'y dy d'x d'y du du' du du' du dx'' /dx\' ■ dx \duj dx d'y dy d'x du du' du dAi' Idx y \du] This, again, being a function of u, further derivatives with respect to x may be obtained by a repetition of the process. EXERCISES. Find the second derivative of x with respect to y, and also of y with respect to x, when the relation of x and y is given by the following equations: 1. a; = a cos m; y^h sin u. 2. x = a cos 2m; y — I sm u. 3. a; = a cos 2m; y = 5(cos u — sin u). 4. a; = M — e sin m; y = m + e sin m, 5. x^e"; y^ tce?\ 94 THE DIFFERENTIAL CALCULUS. 6. Show that if ., d'u 3 sin u « = e" cos M, then ^-5- = -^, = a- ^ ay e*"(cos u — sm u) 7. Show that the wth derivative of a;" + aa;"~' + 5a;"~' is n\, n being a positive integer > 1. 8. Show that X>^'(m') = ^vJ'D^^u + ISuD^uDJ'u + 6(Z>^m)'. 9. Show that if v = m", then DJ'v - nu^-^DJu + 3n(n - l)u''-^D^uDJ'u + n{n - 1) (w - 2)m"-'(Z)^m)'. 10. If ?* = a cos ma; + 1 sin ma;, show that Z>/m + m'M = 0. Then, by successively differentiating this result, show that, whatever the integer n, i?/ + % + m"i?/M = 0; 11. If M = e* cos a; and v = (f sin x, then D^'u = -2v and D^'v = 2m. Also, i)„*t; 4- 4v = 0; i>^% + 4m = 0. 12. If M = e"'" cos ma; and v = e"* sin ma;, show that the successive derivatives of m and v may always be reduced to the form BJu — Aiu — Bfl; DJv = AtV + BfU, (a) where A and B are functions of m and w. Also, find the values of A^, A^ B^ and B^, and show by differentiating (a) that A^^^ = A,Ai- B,B^; Bt+^ = B,At + A.B^. DEVELOPMENTS IN 8EBIE8. 95 CHAPTER VIII. DEVELOPMENTS IN SERIES. 5 7 . A series is a succession of terms all of whose values are determined by any one rule. A series is called Finite when the number of its terms is limited; Infinite when the number of its terms has no limit. The sum of a finite series is the sum of all its terms. The sum of an infinite series is the limit (if any) which the sum of its terms approaches as the number of terms added to- gether is increased without limit. "When such a limit exists, the series is called convergent. When it does not exist, the series is called divergent. To develop a function means to find a series the limit of whose sum, if convergent, shall be equal to the function. We may designate a series in the most general way, in the form «i + w, + ?*, + .,.+ M„ + u„^i -f . . . , the nth. terms being called m„, 58. Convergence and Divergence of Series. ISTo universal criterion has been found for determining whether any given series is convergent or divergent. There are, however, a great number of criteria applicable to a wide range of cases. Of these we mention the simplest. I. A series cannot le convergent unless, as n becomes in-' finite, the ntli term approaches zero as its limit, For if, in such case, the limit of the terms is a finite quantity a, then each new term which we add will always 96 THE DIFFEUENTIAL CALCULUS. change the sum of the series by at least a, and so that sum cannot approach a limit. As an example, the sum of the series 1 — 1 + 1 —1 + 1 — 1, etc., ad infinitum, will continually change from + 1 to 0, and so can approach no limit, and so is divergent, by definition. II. A series all of whose terms are positive is divergent unless nUn = when w = oo . To prove this, we have first to show that the harmonic series i-\-i-{-i + i+ etc., ad infinitum, is divergent. To do this we divide the terms of the series, after the first, into groups, the first group being the 2 terms I + i, the second group the following 4 terms, the third group the 8 terms next following, and, in general, the nth group the 2" terms following the last preceding group. We shall then have an infinite number of groups, each greater than i. Now, if, for all the terms of the series after the nth, we have nu„ > a {a being any finite quantity), then w. > — , n andw„ + Mm+i+ ...>«(- + - — rT + - — r~o + • • •)• ' ^ \m m + 1 m-\-2 J Because the last factor of the second member of this equa- tion increases to infinity, so does its product by a, which proves the theorem. III. If the terms of a series are alternately positive and nega- tive, continually diminish, and approach zero as a limit, then the series is convergent. Let the series be U^ — U, + U, — U^ + U, — . . . . Then, by hypothesis, U^> U,> U^> u^> . . . . DEVELOPMENTS IN 8EBTE8. 97 Let us put 8n for the sum of the first n terms of the series, n being any even integer, and 8 for the limit of the sum, if any there be. Then this limit may be expressed in either of the forms 8=8„ +(m„ + i-m„ + j) + (m,. + 3-m» + 4) + - • • and 8= 8n + l— (w„ + 8 — M„ + s) — (Mn + 4 — Mn + s) — • • • • Since all the differences in the parentheses are positive, by hypothesis it follows that, how many terms soever we take, the sum will always be greater than 8„ and less than jS^„+i. The difEerence of these quantities is «„ + !, which, by hypothe- sis, approaches zero as a limit. Since the two quantities /S„ and ;S'„+i approach indefinitely near each other from opposite directions, they must each approach a limit 8 contained be- tween them. Graphically the demonstration may be shown to the eye thus; Let the line 08^ represent the sum 8n, when n = 6, O Ss Sb Sj»— S Sn S'o Sr i I I 1 I I FiQ. 11. or any other even number; 08^ the sum 8^, etc. Then every succeeding even sum is greater than that preceding, and every succeeding odd sum is less than that preceding, while the two approach each other indefinitely. Hence there must be some limit 8 which both approach. An example of such a series is 1 1,11,1 ^ (— 1)" of which the wth term is — ^r -zr. We shall hereafter see 3m — 1 that the limit of the sum of this series is in: If we divide the terms into pairs whose sums are negative, the series may be written > 3 JJ a 3-5 7-9 1113 etc. 98 THE DIFFERENTIAL CALOULUS. Pairing the terms so that the sum of each pair shall he posi- tive, the series becomes _LJ__1_ _l_f We may show by the preceding demonstration that these series approach the same limit. IV. If, after a certain finite number of terms, the ratio of two consecutive terms of a series is continually less than a cer- tain quantity a, luhich is itself less than unity, then the series is convergent. Let the mth term be that after which the ratio is less than a. We then have Mn + i < ocu„; M» + 8 < «Mn + l < «X; M» + s < "M„ + a < aX; Taking the sum of the members of these inequalities, we have «„ + l + M„+s + M„ + s+ • • • <(« + «' + «' + • ■ ')u„. But a + or' + or' + • • • is an infinite geometrical progres- cc . . sion whose limit when or < 1 is ^ , .a finite quantity. 1 — a Hence, putting S for the limit of the sum of the given series, we have 1 — a The second member of this inequality being a finite quantity which S can never reach, 8 must have some limit less than that quantity. As an example, let us take the exponential series DEYEL0PMENT8 IN 8EBIEB. 99 The ratio of the {n + l)st to the wth term is -. This ratio becomes less than unity when n> x, and it approaches zero as a limit. Hence the series is convergent for all values of a;. CoKOLLAKT. A Series fl„ + a^x + a^x' + a^x' + . . . proceeding according to the powers of a variable, x, is conver- gent when x <1, provided that the coefficients a„ do not in- crease indefinitely. Rbmakkb. — (1) Note that, in applying the preceding rule, it does not suffice to show that the ratio of two consecutive terms is itself always leas than unity. This is the case in the harmonic series, but the series is nevertheless divergent. The limit of the ratio must he less than unity. (3) If the limit of the ratio in question is greater than unity, the series is of course divergent. Hence the only case in which Rule IV. leaves a douht is that in which the ratio, heing less than unity, approaches unity as a limit. But most of the series met with come into this class. (3) The sum of a limited number of terms of a series gives no certain indication of its convergence or divergence. If we should compute the successive terms in the development of e-'™ we should soon find our- selves dealing with numbers having thirty digits to the left of the deci- mal-point, and still increasing. But we know that if we should continue the computation far enough, say to 1000 terms, the positive and negative terms would so cancel each other that in writing the algebraic sum we should have 42 zeros to the right of the decimal-point. On the other hand, if the whole human race, since the beginning of his- tory, had occupied itself solely in computing the terms of the harmonic series, the sum it would have obtained up to the present time would have been less than 44. For 1000 million of people writing'5000 terms a day for 3 million of days would have written only 10^' terms. It is a theorem of the harmonic series, which we need not stop to demonstrate, that T> ^ -..T 1 ^n,^ comm. log 10" 19 But Nap. log 10" = 0.4343 . . ' = .4334^: = *^ ^^' and yet the limit of the sum of the series is infinite. 100 TBE DIFFERENTIAL CALCULUS. 59. Maclaurin's Theorem. This theorem gives a method of developing any function of a variable in a series proceed- ing according to the ascending powers of that variable. If X represents the variable, and (j) the function, the series to be investigated may be written in the form cl>{x) = A,-\-A,x + A,x'-\-A,x'+...; (1) the series continuing to infinity unless is an entire func- tion, in which case the two members are identical. Whether the development (1) is or is not possible depends upon the form of the function <p. Most functions admit of being so developed; but special cases may arise in which the development is not possible. Moreover, the development will be illusory unless the series (1) is convergent. Commonly this series will be convergent for values of x below a certain mag- nitude, often unity, and divergent for values above that mag- nitude. What we shall now do is to assume the development possible, and show how the values of the coefiBcients A may be found. Let us form the successive derivatives of the equation (1). We then have 0(a;) = -^o + ^i« + ^,a;' -f etc.; g= <P'{x) = A, + ZA,x-\.^A,x^+...; ^ = <l>"{x) = 1-2A, + 2-3A,x + 3-iA,x' + . . . ; -f = c^"'{x) = 1-2-3A, + 2-3-iA,x + . dx ^ = 0C)(a;) = 1-2-3-4 . . . wJ„ + etc. By hypothesis these equations are true for all values of x small enough to render the series convergent. Let us then put a; = in all of them. We then have DEVELOPMENTS IN SERIES. 101 ^{0)=A,; .•.^„ = 0(O). <P'(0)=A,; .-.A, = <f>'iO). 0"(O) = l-2^,; ...^, = -l0"(O). ,p"'{0) = 1-2-3A,; .•.A, = :rir5<P"'{0). 1 2^ 1 i-a-s"* 0W(O) = n]A„; .-. A^ = ^0<"'(O). By substituting these values in (1) we shall have the re- quired development. Noticing that the symbolic forms (p'{0), <p"{0), etc., mean the values which the successive derivatives take when we put a; = after differentiation, we see that the coefficients are obtained by the following rule : Form the successive derivatives of the given function. After the derivatives are formed, suppose the variable to be zero in the original function and in each derivative. Divide the quantities thus formed, in order, by 1; 1; 1"3; l'3-3, etc., the divisor of the nth derivative being n\ The quotients will be the coefficients of the powers of the variable in the development, commencing with the zero power, or absolute term. EXAMPLES AND EXERCISES. I. To develop {a + a;)" = w in powers of x. We have u = {a-\- xY; . • . ^„ = a". ~ = n{a + xy~^; .•.A,=na''-\ d'u , ,,,,%.., i n{n — l) „ „ -^=n{n-l){a + xY-^; -•-A= \.c^ ^ «""'- -^= '^^^ - '^) ■ • • ^^ - ^ -^'^'> ^'^ + ^y 102 TEE DrFFEBENTIAL OALGTILXTS. Thus the development is (a + xY = a" + «a»-'a; + (j)a"-"a;' + (j)«"~°*' + • • • . which is the binomial theorem. 2. Develop (a — a;)" in the same way. 3. Develop log (1 + x). Here we shall have du dx~ 1 ■ 1 + x (1+^r 1. > dx' ~ -(1 + ; .)-- d'u dx' ~ 1-2(1 + a:)-- etc. eto. Noticing that log 1 = 0, we shall find log (1 + a;) = a; - ^" + ia;' - K + 4. Develop log (1 — x). 5. Develop cos x and sin x. The successive derivatives of sin x are cos x, — sin x, — cos x, sin «, etc. By putting x=Q, tliese become 1, 0, — 1, 0, 1, 0, etc. Tlius we find 6. Develop e', where e is the Naperian base. Ans. e« = l+a; + |] + |.J+.... 7. Develop e'". 8. Show that a^ = l + .loga+<i^' + ^^'+.... 9. Deduce e"""" = 1 + * + ^-^+ DEVELOPMENTS IN BEBIEB. 103 10. Develop sin (a + x) and cos (a -\-x) and thence, by com- paring with the results of Ex. 5, prove the formulae for the sine and cosine of the sum of two arcs. Pind first x' x' sin {a-{-x)i= sin a (1 — -j- + . .) + cos a {x — ^+ • ■)• 11. Develop (1 + e")" and show that the result may be re- duced to the form lA M n' -\-n x' w° + 3w° «^ "^3^"^""2^~L2 H 35 3T+* • 12. Develop e* sin x and e°° cos a; and deduce the results e'sinas^a^ + aly-falj -4|j -8|y -... e* cos a; = 1 -f- a; — ^jr^ — 4-i-r — 4^7^+,. . . 2! 4! 5! 13. Develop cos" x. Begin by expressing cos' x in the form J cos 3a! -f 4 cos sb. 14. Develop tan '~%. This case affords us an example of how the process of de- velopment may often be greatly abbreviated. It has been shown that f?-tan<-*>a; 1 , , , . . , x / x J = zr-, — 5 = 1 — a' -+- a;* — a; -f etc. (a) ax 1 -f a;' ^ ' Now assume tan<-"a; = A -\- A^x + A^x" -\- etc. This gives ^•*^^""'^ = A, + 2A,x + 3A,x' + etc. {b) Comparing (a) and (5), we have ^1 = 1; ^s = - h ^. = i; A = - h etc. and A, = A^ = A^ . . . =0. The value of A^ is evidently zero. Hence tan<-«a;=:a;-ia;= + ^a:'-|a;'-f etc. (c) 104 TBE DIFFERENTIAL CALCULUS. 15. Develop sin<~"x. ®"^°® — di = <1 - * )" *' we may develop the derivative and proceed as in the last ex- ample. We shall thus find . ,_ .. X , 1 a;', 1-3 a:', 1-3-5 a;' , , sm<% = - + -.3- + ^^.-+^:^^+etc. 60. Ratio of the Circumference of a Circle to its Diameter. The preceding development of tan^~'^a; affords a method of computing the number ?r with great ease. The series (c) could be used for this purpose, but the convergence would be very slow. Series converging more rapidly may be obtained by the following device: Let a, a', a", etc., be several arcs whose sum is 45° = ^tt. We then have tan {a + a' + a" + etc.) = 1. Let t, t', I", etc., be the tangents of the arcs a, a.', a", etc. If there are but two arcs, a and a', we then have, by the addition theorem for tangents, *'^% = \; or t-\-t' = \-tt'. 1-tt' U there are three arcs, a, a% and a", we replace t' by f + 1" in the last expression, and thus get 1 - ft" t + f + f- tt't" = !-«'- ft" - tt". We now have to find fractional values of t, f and t" of the form — , m being an integer, which will satisfy one of these equations. Unity is chosen as the numerator because the powers of the fraction are then more easily computed. The simplest fractions which satisfy the last equation are *~3' ^ -5' * -8- DEVELOPMENTS IN 8EBIIS8. 105 We then have, from the development of tan ^~'' t, etc.. "~2 3-3' 5-3' 1 ** 5 3-5' ' 5-5' -T- » = or + or' + a". 4 These series were used by Dase in computing it to 200 decimals. A combination yet more rapid in ordinary use is found by determining a and a' by the conditions 1 tan a ■■ 5' AlU — a' = -T- 7t. 4 We then have tan 2a = n f o tan 4a; = jj^; and because a' = 4a — i^r = 4ar — 45°, we have , , _ tan 4a — 1 _ 1 ^^^'^ -tan4a-fl-2r9- Hence we may compute ar thus : _3,_J_, J 1_ p " 5 3-5'"^5-5' 7.5' + ---» 239 3-339= ' 5-339' ' "' 7f = 4a — a'. Ten or eleven terms of the first series, with four of the second, will give it to 15 places of decimals. 106 THM DIFFERENTIAL CALCULUS. 61. In developing functions by Maclaurin's theorem we may often be able to express the derivatives of a certain order as functions of those of a lower order. The process of find- ing the higher derivatives may then be abbreviated by retain- ing the derivatives of lower orders in a symbolic form, so far as possible. EXAMPLES. I. Let us develop u = log (1 + sin x) = <p{x). We now have cos X sm X = sec X — tan x; 1 + sin cB cos X <p"{x) = sec X tan x — sec" a; = — sec a;0'(a:). Now, in continuing the differentiation, we use the last of these forms instead of the middle one. Thus <p"'(x) — — sec a; tan x <p'{x) — sec x<p"{x) = — sec a; tan x 4>'{x) + sec' x<p'{x) = - <p\x)4>"{x). We may now find the successive derivatives symbolically. Omitting the symbol x after 0, we have 0'" = — 0'0"' — 0'"; (jf = — (p'cf)^^ — 30"0'"; (ff^ = — (p'(f>^ — 4^"0'^ — 3(f>"". etc. etc. Supposing x = 0, •^(0) = 0; 0'-(O) = - 2; 0'(O) = + 1; 0-(O) = + 5; 0"(O) = - 1; 0-(O) ^ - 16; 0"'(O) = + 1; etc. etc. ence log (1 -j- sin a;) = a; — a;' 2 x' x' x' x' "•"6 12+34 45 "^ DEVELOPMENTS IN SERIES. 107 2. To develop u = tan x. Let us write the equation in the implicit form u cos X — sin X = 0. Then, by differentiation and diyision by cos x, we find B^u = 1 + m'; D^'u = 2uD^u = 2u + 2u'; BJu = 2uDJu + SD^uDJ'u + 6{DJ'uy. Putting M = 0, we find the even derivatives to vanish and the odd ones to become 1, 2, 16, etc. Hence tan x = x -^^x' -{• ^x^ + . . . . 3. To develop m = sec x. Differentiating the form w cos cb — 1 = 0, we find DjM cos a; — M sin a; =: 0. (a) The successive derivatives of this equation may each be written in the form M cos X — N sin a; = 0. (5) For, if we differentiate this equation with respect to x, it becomes {D^M - N) cos X- {M+ D^N) sin a; = 0. Hence the derivative of (5) may be formed by putting M' = D^M- N; N' = M-\-B^N, (c) and writing M' and N' instead of M and N in the equation. In {a) we have M = D^u; N='U,. Then, by successive substitution in (c), Jf = D^'u- u; N' - 2D^u; M" - D^'u- 3D^u; N" = SD^'u - u; M'" = DJii- en^'u + u; N"' = 4J)J'u - iD^u; M'^ = DJu-lODJ'u + 5D^u; W'" - 6DJ'u - lODJ'u + u. M" = DJu - IbD^'u + 152)^»M - u; 108 THE BIFFEBENTIAL OALOULm. When a; = 0, we have sin a; = 0, cos x = l, u = 1, and hence M = M' = . . . = in all the equations. Thus we find, for X = 0, DJu = u = 1; D^'u = 6-1 = 5; D^'u = 75 - 15 + 1 = 61; etc. etc. ; while the odd derivatives all vanish. Hence ^ , 1 . . 5 . , 61 , , sec a; = 1 + - k' + j-j a; + gj a; + 63. Taylor's Theorem. Taylor's theorem differs from Maclaurin's only in the form of stating the problem and ex- pressing the solution. The prohlem is stated as follows: Having assigned to a variable z an increment h, it is re- quired to develop any function of x-\-h in powers of h. Solution. Let be the function to be developed, and let nsput u = 0(a;); | M' = 0(a;+A). I Assume u' = X„ + XJi + X,h' + XJi' + etc where X„, X^, etc., are functions of x to be determined. Then, by successive differentiation, we have du/ dh d*u' (1) (3) dh' d'u' dh' etc. : X, + ZXJi + ZXJi' + 4X.7j' + etc.; : 3X, + 2 • ZX,h + 3 • 4X,/^' + etc. ; (3) 1-3-3X, + 3-3-4X,A + etc. etc. etc. We now modify these equations by the following lemma: If we have a function of the sum only of several quantities, the derivatives of that function with respect to those quantities will be equal to each other. DEVELOPMENTS IN SEIIIE8. 109 For if in f{x + /*) we assign an increment /lA to x and to h separately, the results will be /(a; -\-h-{- Ah) and /(a; -f Ah + h), which are equal. It follows that we have du' _ du' dh~ dx' Now these equal derivatives, like u' itself, are functions of x-\-h alone, so the lemma may be applied to as many suc- cessive derivatives as we please, giving d'u' _ d'u\ d¥ ~ dx^ ' ffu' _ d'u' d¥ ~ dx' ' etc. etc. Now let the derivatives with respect to x be substituted for those with respect to h in equations (3), and let us suppose h to become zero in equations (2) and (3). Then u' and its de- rivatives will reduce to u and its derivatives, and we shall get -p. _ du ^■-^' „ _ J_ ^ '~l-2dx" Y - 1 ^'^ ''~l-2-idx''' _ 1 d"u ''~ n\ dx"" ' Then, by substitution in (3), we shall have, for the required development, , , du h , d'u V , d^u ¥ , , " ='* + 5^r+;to^r2+^'FF3+^*«- This formula is called Taylor's Theorem, after Brook Taylor, who first discovered it. 110 THE DIFFERENTIAL CALCULUS. EXAMPLES AND EXERCISES. 1. Develop {x -\- /*)». We proceed as follows : u = a;"; du dx ^^,=n{n- 1) {n - %-'; etc. etc. By substitution in the general formula we find {x + hy = a;" + ^a"-' h + ^^" ~ ^^ a;"-" A 1 L' Z n(n - 1) (w - a) 3,3 , 2. Develop the exponential function a " + * in powers of h. Ans. a^il + log a^ + (log a)'^-^ +.••]• 3. sin (x -\- h). 4. cos {x -\- h). 5. sin {x — h). 6. cos (a; — A). 7. log (ce + h). 8. log (a; — A). , a; + 7i 9. log T. 10. log cos a;. iJ/ "^ /J II. cos' {x 4" A). 12. sin' {x — h). 13. tan<-"(a; + A). 14. sin (-'> (a; - 7i). 15. Deduce the general formula 1 6. Prove, by differentiation and appljang the algebraic theorem that in two equal series the coefficients of like powers of the variables must be equal, that if we have log («o + «i» + o,a;' + ...) = J„ + 5,a; +5,a;' + . . . , DEVELOPMENTS IN SERIES. Ill then the coefficients a and b are connected by the relations \ = log «o; a A = «>; %a,\ + afi, = 2a,; 3a^b, + 2a^b, + a,b, - 3a,; etc. etc. etc. 17. Hence show that is the logarithm of the sum of an infinite series whose first terms are 3a;' , 13a;' , 73a;* , \ e(l + x + '' 2 ' 6 ' 34 ' • ■ V 63. Identity of Taylor's and Maclaurin's Theorems. These two theorems, though different in form, are identical in principle. To see how Taylor's theorem flows from Maclaurin's, notice that Ti in the former corresponds to x in the latter. The de- rivatiyes with respect to x in Taylor's theorem are the same as the derivatives with respect to h, and if we suppose /» = after differentiation Taylor's form of development can be de- rived at once from Maclaurin's. Conversely, Maclaurin's theorem may be regarded as a special case of Taylor's theorem, in which we take zero as the original value of the variable, and thus make the increment equal to the variable. That is, if we put f{x) in the form /(O + x), and then, using x for li, develop in powers of x by Taylor's theorem, we shall have Maclaurin's theorem. 64. Gases of Failure of Taylor's and Maclaurin's Theorems. In order that a development in powers of a vari- ble may have a determinate value it is necessary that none of the coefficients in the development shall become infinite and that the developed series shall be convergent. Por example, cosec x cannot be developed in powers of x, because when x = the cosecant and all its derivatives be^ come infinite. 112 THE DIFFERENTIAL CALGULUS. 65. Extension of Taylor's Theorem to Functions of Several Variables. Let us have the function w =/(», y). (1) It is required to develop this function when x and y both re- ceive increments. Let us first assign to x the increment 7i, and suppose y to remain constant. We then have, by Taylor's theorem, . . du h d'u ¥ d'u h' . . in which u, t— , etc., are all functions of y. Next, assign to y the increment k. The first member of (2) will become /(a; -}• 7i, y -\- k). Developing the coefficients in the second member in powers of k, the result will be: u will be changed into du h d'u k' d'u ¥ " + 5^ r + ^' 2 ! + ^= 3 ! + • • • ' -7— = D^^u will be changed into „ d-D^uh d'Dxu¥ ■^"■^ '^'~d^l^~df 2! + • • • ^ d'u ^-i = D^u will be changed into „, , d-D^u lc , d'DJu lc' , ■^'='* + ~^%~l+~^'2!+--- 5 etc. etc. etc. Substituting these changed values of the coefficients in (2) it will become ., , , , , , , du k , d'u ¥ , d^u ¥ , duh d'u li Tc d'u h ¥ '^dxl^ dxdy 1 1 "*" dxdf 1 2 ! + ' ' ' d'u h' d'u ¥ h d'u ¥¥ + dx' 2! "^ dx'dy^l 1 + ^3;"%" 2! 2! '^ dx'd\^ DEVELOPMENTS IN SERIES. 113 Thus the function is developed in powers and products of the increments h and Ic. The law of the series will be seen most clearly by using the i?-notation. For each pair of positive and integral values of m and n we shall have the term ml n\ If we collect in one line the terms of the development which are of the same order in It and k, we shall have: Order of Terms. , , 1st. Dji- + DyUj. 2d. i),'M|j + D^D,u\-\ + D,'u^,. 3d. D^'u^, + D^'D.u Ij ^ + D^D^u ||j + D.'u |j; rth. X>/wi.J+ i),'-ii)„M^_l^ _ + EXERCISES. 1. Show that in the preceding development the terms of the rth order may be written in the form ^ I nrD^ru + ^|^^|^r - 1 ^.D^r - IJJ^^ + (^^i •- 'A'^^x*- " ^Dy^V, + . . . L \t)' (9")' ^^^'' •isnoting the binomial coeflScients as in § 5. 2. Extend the development to the case of three independent variables, and show that the t??ni? to the second order in- clusive will be as follows : 114 If THE DIFFERENTIAL CALCULUS. u=f{x,y, Z), then /■{x-\-h,y-\-k,z-\-l)=u + D^u-h + DyU-k + D^u-l + DjiD^u • hi + DyUD,u ■ kl. 66. Hyperiolic Functions. The sine and cosine of an imaginary are may be found as follows: In the deyelopments for sin x and cos x, namely. sm X ■■ X' , X' cos a; = 1 3! ^5! a;' «* _ 2! ' 4! let us put yi for x. (i = V— 1). We thus have ■)■■ sin yi = i\^i + U.J + Ij + cos yi = 1 + 1; + f I + • (1) We conclude: The cosine of a purely imaginary arc is real and greater than unity., while its sine is purely imaginary. We find from (1), cos yi + i sm yi = 1 — y -\-^. — etc, = e""; COS yi — i sin 2/i = 1 + y + |-, + etc. = e"; and, by addition and subtraction, COS yi = \(e~'' -\- e*); i sin «/i = ■J(e~'' — e"); sin yi = ^i{e^ — e"*). The cosine of yi is called the hyperbolic cosine of y, and is written cosh y, the letter h meaning "hyperbolic." DEVELOPMENTS IN SERIES. 115 The real factor in the sine of yi is called the hyperbolic sine of y, and is written sinh y. Thus the hyperbolic sine and cosine of a real quantity are real functions defined by the equations sinh?/ = ^{6" — e—");\ coshy = i(e''H-e-''). 5 By analogy^ we introduce the additional function tanh y ■■ (1) e^ + e-" The differentiation of these expressions gives d sinh y , d cosh y . , ,„, — ^-■- = cosh 2/; -^-l=smhy; (2) d tanh y — — cosh" y' They also give the relations cosh" y — sinh" y = 1. (3) Inverse Hyperbolic Functions. When we form the inverse function, we may put u = cosh y. Then, solving the equation el -\- e-" = 2 cosh y = 2u, we find e" = u± V-u Hence y = log (u ± Vu' — 1) = cosh ^~*' M. (4) In the same way, if we put u = sinh y, we find y = log («. ± V'u' + 1) = sinh<-« u. (5) From the equations (3) and (3) we find, for the derivatives of the inverse functions: 116 THF, DIFFERENTIAL CALGULUi When y — cosh'~'> u, or u = cosh y, 1/1x611 dy _ 1 du Vw> _ 1 When y — sinh'~^' u, or ^t, = sinh y, then dy _ 1 au ^u' + 1 (6) (7) Eemaek. The above functions are called hyperbolic be- cause sinh y and cosh y may be represented by the co-ordinates of points on an equilateral hyperbola whose semi-axis is unity. The equation of such an hyperbola is x'-y' = 1, which is of the same form as (3). EXERCISES. 1. By continuing the differentiation begun in (3) prove the following equations: Dj.' sinh X = sinh x; DJ' cosh X = cosh x; Dx^~^ sinh x = sinh x. etc. etc. 2. Develop sinh x, as defined in (1), in powers of x byMac- laurin's theorem. Ans. sinh a; = y + |-j -f- |j + . . . . 3. Develop sinh (x -\- h) and cosh {x -\- h) by Taylor's theorem and deduce sinh (x+h) = sinh a;f 1 + ^, + . . .]+ cosh x\x+^^+ . . . j = sinh X cosh h + cosh x sinh h; cosh (x+h) — cosh x cosh h + sinh x sinh h. MAXIMA AND MINIMA. Ill CHAPTER IX. MAXIMA AND MINIMA OF FUNCTIONS OF A SINGLE VARIABLE. 67. Def. A maximum value of a function is one wMch is greater than the values immediately preceding and follow- ing it. A minimum value is one which is less than the values immediately preceding and following it. Eemaek. Since a maximum or minimum value does not mean the greatest or least possible value, a function may have several maxima or minima. 68. Peoblem. Having given a function y = <P{«), a is required to find those values of x for which y is a maxi- mum or a minimum. Let us assign to x the increments -j- h and — h, and develop in powers of h. We shall then have , , , , . dy h , d'y ¥ , ti -1/ , 7N , dy h , d'y h' , , In order that the value oi y = 4>{x) may be a minimum, it must, however small we suppose h, be less than either y' or y". That is, the expressions , dy 7i , d'y V y -y=-d^i+ii^iii-^^-' ,, , dy h . d'y h" , , 118 TEE DIFFERENTIAL CALCULUS. must both be positiye as h approaches zero. But if -^ is finite, h may always be made so small that the terms in A' shall be less in absolute magnitude than those in h (§ 14), and the condition of a minimum cannot be satisfied. We must therefore have, as the first condition, |^0'(-) = O. (1) By solving this equation with respect to x will be found a value of X called a critical value. The same reasoning applies to the case of a maximum, so that the condition (1) is necessary to either a maximum or a minimum. Supposing it fulfilled, we have d'y h' d'y ¥ , , y -y=i^^v2-di^T^+^^-' y y- dx'i-^i^ dx'i-%-d^^^^- Since A' is positive, the algebraic sign of these quantities, as li approaches zero, will be the same as that of -t4' When this second derivative is positive for the critical value of X, y, being less than y' or y", will be a minimum. When negative, y will be greater than either y' or y", and so will be a maximum. We therefore conclude: Conditions of mininium: -j— = 0; -^^ positive. Conditions of maximum: -^=0; -^.^ negative. We have, therefore, the rule: Eq'uate the first derivative of the function to zero. Tliis equation will give one or more values of the independent vari- able, called critical values, and thence corresponding values of the function. MAXIMA AND MINIMA. 119 Suhstituie the critical values in the expression for the second derivative. When the result is positive, the function is a minimum; when negative, a maximum. Exceptional Cases. It may happen that the second deriva- tive is zero for a critical value of x. We shall then have d'y ¥ , d'y h' d'y V , d'y h' , , : 0. If this condition is fulfilled, y will be a maximum and there can be neither a maximum nor a minimum unless dhi dx'' when the fourth derivative is negative; a minimum when it is positive. Continuing the reasoning, we are led to the following ex- tension of the rule : Find the ■first derivative in order which does not vanish for a critical value of the independent variable. If this de- rivative is of an odd order, there is neither a maximum nor a minimum; if of an even order, there is a minimum when the derivative is positive, a maximum when it is negative. The above reasoning may be illustrated by the graphic rep- resentation of the function. When the ordinate of the curve is a maximum or a minimum the tangent will be parallel to the axis of abscissas, and the angle which it makes with this axis will change from positive to negative at a point hav- ing a maximum ordinate, and from negative to posi- tive at a point having a minimum ordinate. For example, in the fig- -^^- '^^• ure a minimum ordinate occurs at the point Q, and maxi- mum ordinates at P and R. 120 THE DIFFERENTIAL 0ALCULU8. EXAMPLES AND EXERCISES. I. Find the maximum and minimum values of the expres- sion y = %x'-{- Bz' - 36a; + 15. By differentiation, ^ == 6a;' + 6a; - 36; ax Equating the first derivative to zero, we have the quadratic equation a;' + a; — 6 = 0, of which the roots are x = 2 and a; = — 3. d'x The values of -^—^ are + 30 and — 30. Hence x = 2 gives a minimum value of y = — 29; • a; = — 3 gives a maximum value ot y = -\- 95. Find the maximum and minimum values of the following functions: 2. x' + 3a;' — 242; + 9. 3. x' — 3x + 5. X x' — X -\-l log X log X 8. y = af . 9- y = sin 2a; — x. 10. y = {x-\- l){x - 2)'. II. y = {x- a'Xx - h)'. "•^-(a;+2r '3- y (x - p){x - ^Y 14. y = cos 2a;. 15. y = cos wa;. 16. y = sin 3a;. 17. y = sin wa;. a; Ans. A maximum when a; = -f-cos x. ' ^ ~ 1 -f- a; tan x' A minimum when a; =— cos a; MAXIMA AND MINIMA. 121 19. y = sm X cos x. 20. y = sm x cos a;, sin a; cos x 1 + tan x' '—3 J _j_ ^^Q g." 23. The sum of two adjacent sides of a rectangle is equal to a fixed line a. Into what parts must a be divided that the rectangle may be a maximum? Ans. Each part = ^a. Note that the expression for the area is «(« — x). 24. Into what parts must a number be divided in order that the product of one part by the square of the other may be a maximum? Ans. Into parts whose ratio is 1 : 3. Note that if a be the number, the parts may be called x and a — x. 25. Into what two parts must a number be divided in order that the product of the with power of one part into the nth. power of the other may be a maximum? Ans. Into parts whose ratio is m : n. 26. Show that the quadratic function ax^ -\-'bx-\-c can have but one critical value, and that it will depend upon the sign of the coeflScient a whether that value is a maximum or a minimum. 27. A line is required to pass through a fixed point P, whose co-ordinates are a and b in the plane of a pair of rectangular axes OX and OY. What angle must the line make with the axis of X, that the area of the triangle XTO maybe a minimum? Show also that P must bisect the segment XY. Express the intercepts which the line cuts off from the axes in terms of a, b and the variable angle a. The half product of these intercepts will be the area. We shall thus find Fio. 13. 9 Area = (d + 5 cot a:)(5 + a tan or) = 3aJ-j- a^ tan a -f- tan a 122 THE DIFFERENTIAL CALCULUS. Then, taking tan a — ta& the independent variable, we readily find, for the critical values of t and a, J t = ± - , or assinct=±J cos a. a It is then to be shown that both values of t give minima values of the area ; that the one minimum area is Zab, and the other zero ; that in the first case the line TX is bisected at P, and in the other case passes through 0. 28. Show by the preceding figure that whatever be the an- gle XO Y, the area of the triangle will be a minimum when the line turning on P is bisected at P. The student should do this by drawing through P a line making a small angle with XPT. The increment of the area XOT will then be the difference of the two small triangles thus formed. Then let the small angle become infinitesimal, and show that the increment of the area XOFcan become an infinitesimal of the second order- only when PX= PT. 29. A carpenter has boards enough for a fence 40 feet in length, which is to form three sides of an enclosure bounded on the fourth by a wall already built. What are the sides and area of the largest enclosure he can build out of his ma- terial? Ans. 10 X 30 feet = 300 square feet, 30. A square piece of tin is to have a square cut out from each corner, and the four projecting flaps are to be bent up so as to form a vessel. What must be the side of the part cut out that the contents of the vessel may be a maximum? Ans. One sixth the side of the square. 31. If, in this case, the tin is a rectangle whose sides are 2a and 2i, show that the side of the flap is i{a + b - Va' -ab + b'). 32. What is the form of the rectan- gle of greatest area which can be drawn in a semicircle? Note that if r be the radius of the circle, and X the altitude of the rectangle, l/r^ — x^ will be half the base of the rectangle. Fig. 14. MAXIMA AND MINIMA. 123 69. Oase when the function which is to be a maximum or minimum is expressed as a function of two or more variables connected by equations of condition. The function which is to be a maximum or minimum may be expressed as a function of two variables, x and y, thus: u = 4>{x, y). (1) If X and y are independent of each other, the problem is different from that now treated. If between them there exists some relation A^, y) = 0, (2) we may, by solving this equation, express one in terms of the other, say y in terms of x. Then substituting this value of y in (1), u will be a function of x alone, which we may treat as before. It may be, however, that the solution of the equation (2) will be long or troublesome. We may thfen avoid it by the method of § 41. Erom (1) we have du _ ldu\ fdu\dy dx ~ \dx J \dy Jdx ' and from (3) we have, by the method of § 37, dy _ BJ dx ~ Dyf Substituting this value in the preceding equation, we shall have the value of -^, which is to be equated to zero. The equation thus formed, combined with (2), will give the critical values of both x and y, and hence the maximum or minimum value of u. 124 THE DIFFERENTIAL CALCULUS. EXAMPLES AND EXERCISES. I. To find the form of- that cylinder which has the maxi- mum volume with a given extent of surface. The total extent of surface includes the two ends and the convex cylindrical surface. If r be the radius of the base, and h the altitude, we shall have : Area of base, itr'<. Area of convex surface, %7crh. Hence total surface = 27r(r' + rh) = const. = a. (a) Also, voliune = ttj-'/i. (6) Putting M for the volume, we have, from (J), du n , , „dh -T- = 27(rh + nr^^-. dr dr I*rom (a) we find dr~ r ' Whence -^ = Ttrh — 2«r*. dr Equating this to zero, we find that the altitude of the cylinder must be equal to the diameter of its base. 2. Pind the shape of the largest cylindrical tin mug which can be made with a given weight of tin. This problem differs from the preceding one in that the top is sup- posed to be open, so that the total surface is that of the base and con- vex portion. Ans. Altitude = radius of bottom. 3. Find the maximum rectangle which can be inscribed in a given ellipse. If the equation of the ellipse is bV + aV = a''b\ the sides of the rectangle are Sa; and 3y. Hence the function to be a maximum is 4a;^, subject to the condition expressed by the equation of the ellipse. This condition gives dp _ h^x das ~ a^y MAXIMA AND MINIMA. 125 We shall find the rectangle to be a maximum when its sides are proportional to the corresponding axes of the ellipse; each side is then equal to the corresponding axis divided by yS. 4. Find the maximum rectangle which can be inscribed in the segment of a parabola whose semi- parameter is p, cut ofE by a double ordinate whose distance, OX, from the vertex is a. Show also that the ratio of its area to that of the circum- scribed rectangle is con- stant and equal to 2: V27. By taking x and y as in the Fio. 15. figure, a — X will be the base of the rectangle, and we shall have 2y for its altitude. Hence its area will be 32/(a — x), while x and y will be connected by the equation of the parabola, y'^ = 2px. 5. Find the cone of maximum volume which shall have a given extent of conical surface. Ans. Alt. = radius of base X V2. 6. Find the volume of the maximum cylinder which can be inscribed in a given right cone, and show that the ratio of its volume to that of the cone is 4 : 9. 7. Find the cylinder of maximum cylindrical surface which can be inscribed in a right cone. Ans. Alt. of cylinder = ^ alt. of cone. 8. Find the maximum cone which can be inscribed in a given sphere. If we make a central section of the sphere through the vertex of the cone, the base and slant height of the cone will be the base and equal 126 THE DIFFERENTIAL CALCTTLTTS. sides of an isosceles triangle inscribed in the circular section. Thus the equation between the base and altitude of the cone can be obtained. Ans. Alt. = f radius of sphere. 9. Find the maximum cylinder which can be inscribed in an ellipsoid of revolution. Ans. Alt. = —p of axis of revolution. 10. Find the cone of maximum conical surface which can be inscribed in a given sphere. r I. Of all cones having the same slant height, which has the maximum volume ? 12. A boatman 3 miles from the shore wishes, by rowing to the shore and then walking, to reach in the shortest time a point on the beach 5 miles from the nearest point of the shore. If he can pull 4 miles an hour and walk 5 miles an hour, to what point of the beach should he direct his course? Ans. 4 miles from the nearest point of the shore. Express the whole time required in terms of the distance x of his point of landing from the nearest point of the shore. 13. Find the maximum cone which can be inscribed in a paraboloid of revolution, the vertex of the cone being at the centre of the base of the paraboloid. Ans. Alt. = \ alt. of paraboloid. 14. Find the maximum cylinder which can be described in a paraboloid of revolution. 15. Find the rectangle of maximum perimeter which can be inscribed in an ellipse. 16. On the axis of the parabola y' = 2px a point is taken at distance a from the vertex. Find the abscissa of the near- est point of the curve. Begin by expressing the square of the distance from the fixed point to the variable point (a;, y) on the parabola. 17. Determine the cone of minimum volume which can be circumscribed around a given sphere. MAXIMA AND MINIMA, 127 1 8. Determine the cone of minimum conical surface which can be circumscribed around a given sphere. 19. Find that point on the line joining the centres of two circles from which the greatest length of the combined cir- cumferences will be visible. 20. Find that point on the line joining the centres of two spheres of radii a and h respectively from which the greatest extent of spherical surface will be visible. Ans. The point dividing the central line m the ratio «' : h . 21. Show that of all circular sectors described with a given perimeter, that of maximum area has the arc equal to double the radius. 22. A ship steaming north 13 knots an hour sights an- other ship 10 miles ahead, steaming east 9 knots; What will be the least distance between the ships if each keeps on her course, and at what time will it occur? Ans. Time, 32 min.; distance, 6 miles. 23. What sector must be taken from a given circle that it may form the curved surface of a cone of maximum volume? Ans. Vf of the circle. 24. A Norman window, consisting of a rectangle sur- mounted by a semicircle, is to admit the maximum amount of light with a given perimeter. Show that the base of the rectangle must be double its altitude. 128 THE DIFFERENTIAL CALCULUS. CHAPTER X. INDETERMINATE FORMS. 70. Let us consider the fraction 0(^) = ^| (1) For any value we may assign to x there will be a definite value of (p{x) found by dividing the numerator of the frac- tion by the denominator. To this statement there is one exception, the case of a; = 3. Assigning this value to x, we have 0(3) = f. Now, the quotient of two zeros is essentially indeterminate. For the quotient of any two quantities is that quantity which, multiplied by the divisor, will produce the dividend. But any quantity whetever when multiplied by will pro- duce 0. Hence, when divisor and dividend are both zero, any quantity whatever may be their quotient. But when we consider the terms of the fraction, not as ab- solute zeros, but as quantities approaching zero as a limit, then their quotient may approach a definite limit. We then regard this limit as the value of the fraction corresponding to zero values of its terms. As another example, consider the quantity We may compute the value of this expression for any value of X except 2. When x = % the terms will both become in- finite. Since if any quantity whatever be added to an infinite INDETERMINATE P0BM8. 129 the sum will be infinite, it follows that any quantity what- ever may be the difference of two infinites. There are several other indeterminate forms. The follow- ing are the principal ones which take an algebraic form: ^; -; OXoo; oo - oo ; 0°; oo"; 1". 11. Evaluation of the Form -g. In many cases the inde- terminate character of an expression may be removed by algebraic transformation. For example, dividing both terms of the fraction (1) by a; — 3, it becomes x -\- 3, a determinate quantity even for x = 3. Again, the expression (3) can be reduced to the form — — -r, which becomes i when x — 2. x-\-% The general method of dealing with the first form is as follows: Let the given fraction be 0(£) f(x)' and let it be supposed that both terms of this fraction vanish when X — a, so that we have 0(a) = and ^(a) - 0. (3) Put h=x — a, and develop the terms in powers of A by Taylor's theorem. We shall then have 4>(x) = cp{a + h) = 0(«) + A0'(fl) + ^ct>"{a) + . . . ; fix) = i>{a + h) = f{a) + hf'{a) + ^f"{a) + . . . ; whence, for the value of the fraction (comp. Eq. (3)), (4) Now, when h approaches zero as a limit, the value of this fraction approaches 0>) f\a) 130 THE DIFFERENTIAL CALCULUS. as a limit, which is therefore the required limit of the frac- tion when both its members approach the limit zero. It may happen that <p'{d) and ^p'{a) both vanish. In this case the required limit of the fraction in (4) is seen to be ,p"{a)' In general: The required limit is the ratio of the first pair ^/derivatives of like order which do not loth vanish. If the first derivative which vanishes is not of the same order in the two terms, — for example, if, of the two quantities 0'(a) and ^'{a), one vanishes and the other does not, — ^then the limit of the fraction will be zero or infinity according as the vanishing derivative is that of the numerator or denominator. Eemaek. It often happens that the terms of the fraction can be developed in the form (4) without forming the succes- sive derivatives. It will then be simpler to use this develop- ment instead of forming the derivatives. EXAMPLES AND KXERCISES. a;' -a' for X = a.* X — a <f>{x) -x' - a'; <p'(x) = 2x; - • . (p'{a) = 2a; ip{x) =x — a; f{x) = 1; .-. f'{a) = 1. x' — a' . ' . lim. (x = o) = 2a, X — a^ ' a result readily obtained by reducing the fraction to its lowest terms. log a; x-\ e -e X for x = \. Ans. 1. -X — for x = 0. Ans. 2. * Using strictly the notation of limits, we should define the quantity sought as the limit of the fraction when x approaches the limit a. But no confusion need arise from regarding the limit of the fraction as its value for x = a, asis customary. INDETERMINATE FORMS. 131 a; — sin a , 4- i — -- for (x = 0). Ans. ^. Here the successive derivatives of the terms are: ^'(x) = 1 — cos x; tj)"{x) = sin x; (p"'{x) = cos x. ib'lx) = 3x'; i/j"{x) = 6x; f"'{x) = 6. The third derivatives are the first ones which do not vanish for X = 0. Q^ 7)^ ff 5- for a; = 0. Ans. loga— logJ=logj^. X ^ tan a; — sin a; . 6. r for x~V). Ans, 3. a; — sm a; 7. ; '■ for a; = 0. Ans. -,. 1 — cos n% n 8. L- for a; = 1. Ans. a log a. a; — 1 ^ a" Ja: 9- T- for a; = 1. ^«s. a log a — J log 5. sin a; — sin a , , lo. for %■= a, Ans. cos a. X — a tan « — tan a , , see' a II. — r^- r~ for y = a. Ans. ^r—. — . cos y — cos a ^ 3 sm a log (1 + a;) + log (1 - a;) ^ cosi-sef -^ f°^ ^ = «- ^4«^-+l- 13. H(« + ^)-I°g(«-^) fo, ^ = 0. Ans. I a; a sin 2a; + 3 sin' a; — 2 sin a; . 14- 5 for x — 0. Ans. 4. cos X — cos X e°—e-''—2x, - . 15. : for X = 0. Ans. 1. X — sm X , c" + sin « — 1 . „ . „ 1 6. ■ ' .^ f — r— for « = 0. Ans. 2. log (1 + y) 1 — sin a; — cos a; + log (i + a;) . , 17. ^_iJa; *°'' (^ = ®)- ^"«- 0- 132 THE DIFFERENTIAL CALCULUS. 12. Forms — and X oo . These forms may be reduced 00 to the preceding one by a simple transformation. Any frac- tion -=,- may be written in the form ., ' .., . If N and D both become infinite, 1 -=- Z> and 1 -i- N will both become infini- tesimal, and thus the indeterminate form of the fraction will bet Again, if of two factors A and B, A becomes infinitesimal while B becomes infinite, we write the product in the form jj, and then it is a fraction of the first form. But this transformation cannot always be successfully ap- plied unless the term which becomes infinite does so through haying a denominator which vanishes. For example, let it be required to find the limit of a;"'(log xY for a; = 0. Here a;" approaches zero, while log x, and there- fore (log a;)", becomes infinite for x = 0. Hence the denomi- nator of the transformed fraction will be ^ (putting for brevity I = log x). The successive derivatives of this quantity with respect to x are The successive derivatives of the numerator are mx''~'^; m{m — l)x'"~^; etc. The limiting values of the given quantity a;"?" thus become mx'^l"*^ m{m — l)x'" n '' (J- ^ !L±i\ ' etc.. which remain indeterminate in form how far soever we may carry them. INBETEBMINATE F0BM8. 133 In such cases the required limit of the fraction can be found only by some device for which no general rule can be laid down. In the example just given the device consists in replacing a; by a new variable y, determined by the equation log a; = - y. We then have a; = e ~ ". Since for a;i02/=oo,we now have to find the limit of (-y)" ^ / y.n^ for y = OS. By taking the successive derivatives of the two terms of w" the fraction ~, we have the successive forms ny^^_ n{n — l) .y"~^. n{n — 1) {n — 2)y"-^ _ me*"" ' wiV""" ' jw^e*"" ' ^ °' Whatever the value of n, we must ultimately reach an ex- ponent in the numerator which shall be zero or negative, and then the numerator will become « ! if w is a positive integer, and will vanish for y i oo , if «. is not a positive integer. But the denominator will remain infinite. We therefore con- clude: lim. [a:"'(log a;)"] (a; i 0) = 0, whatever be m and oi, so long as m is positive. From this the student should show, by putting z = x~^ and m — 1, that the fraction z (log z)" becomes infinite with z, how great soever the exponent n, and therefore that any infinite numier is an infinity of higher order than any power of its logarithm. 73. Form oo — cxi . In this case we have an expression of the form F{x) = u — V, 134 TEE DIFFERENTIAL CALCULUS. in which both u and v become infinite for some value of x. Placing it in the form we see that F{x) will become infinite with u unless the fraction V — approaches unity as its limit. When this is the case the expression takes the form oo x of the preceding article. 74. Form 1". To investigate this form let us find the limit of the expression when n becomes infinite. Taking the logarithm, we have log u = hn log ^1 + -j Making n infinite, we have lim. log u = h; or, because the limit of log u is the logarithm of lim. u, log lim. u = Ti. I IN*" Hence lim. 1 1 + - 1 (w = oo ) = e*. In order that this result may be finite, h itself must not be infinite. We therefore reach the general conclusion: Theobbm. In order that an expression of the form (1 + «)' may have a finite limit when a becomes infinitesimal and z infinite, the product ax must not become infinite. Cor. If the product ax approaches zero as a limit, the given expression will approach the limit unity. inbetehminate forms. 135 75. Forms 0" and oo°. Let an expression taking either of these forms as a limit be represented by u'^=F. The problem is to find the limiting value of the expression when (p approaches zero and u either approaches zero or becomes infinite. From the identity w = e'"*" we derive ^ = m* = e* '■« ". We infer that the limit of F will depend upon that of log m. If lim. <f) log M is + °° > then lim. F= co. If lim. log M is — 00 , then lim. F=0. If lim. <p log M is 0, then lim. F= 1. If lim. log u is finite, then lim. F is finite. Hence the rule: To find the limit of m* when i and w — or 00 , put I £ lim. ^ log u. Then lim. M* — e'. EXAMPLES AND EXERCISES. 1. Find lim. af for x = 0. Here ar" = e"" ">«'". Since z log a; has zero as its limit when x = 0, the required limit is e° or 1. 2. lim. a;"^ for x = 0. Ans. F=l. 3. lim. x" for a; i 00. 1 4. a;'-"" for x = l. n 5. a;!-" for a; = 1. h 6. (1 — a;)^ for x = 0. 7. = 7^-j — r for a; = 0. log (1 + x) „ log sin 2a; . . > 1 o 8. -~ — -. for a; = 0. Ans. log 2. log sin X e- + log(l-^) zJ: fo, ^^0. ^«s. i. • a; — tan a; Ans. F=l. Ans. 1 e ■ Ans. «-". Ans. e-\ Ans. 2. 136 THB DIFFERENTIAL OALGTJLTJB. ' ° ' for a; = 00. m" . Tt It II. z tan a; — — sec a; for x = rr-. . a , 12. y sin — for y z= <x>. 13. x\a^ — l) for a; = 00 . for a; = 0. 1 for x — 0. Ans. 1. Ans. - 1. Ans. a. Ans. logo. Ans, 1. Ans. et. Ans. e-K Ans. 2 n /tan a;\ /tan a;\x» 16. (cos a;)x' for a; = 0. 17. (1 - y) tan |«/ for y = \. 1 18. ^^_2SfV for a; = 0. ^ws. 1. 19. a; — a;' log f 1 -| j for a; = 00. ^ws. \. gr g - a; 20. = T- r- for a; = 0. ^ms. 2. log (1 + X) 2 ' T^ ' J f or a; = 0. Ans. a^a^. n 22. (-J — ■ — ^— ' ■ — ^1 for a; = 0. Ans. a^a,...an. 23, Show that, how great soever the exponent n, 7i r- iz 00 when a; i 00 . (log a;)" PLANE CUBVBa. 137 CHAPTER XI. OF PLANE CURVES. "76. Forms of the Equations of Curves. As we have here- tofore considered curve lines, they have been defined by an equation between the co-ordinates of each point of the curve, and therefore of one of the forms y=f{x); x=f{yy, (1) and F{x, y) = 0. The distinguishing feature of the equation is that when we assign a value at pleasure to one of the co-ordinates x or y, one or more corresponding values of the other co-ordinate are determined by the equation. But the relation between x and y may be equally well defined by expressing each of them as a function of an auxiliary variable, which is then the independent variable. Calling this auxiliary variable u, the equations of a curve will be of the form y = <p,{n). f ^^> Assigning values at pleasure to u, we shall have correspond- ing values of x and y determining each point of the curve. An advantage of this method of representation is that for each value of u we have one definite point of the curve, or several definite points when the equations give several values of the co-ordinates for each value of u; and we thus have a relation between a point and the algebraic quantity u. It is also to be remarked that by eliminating u from the equations (3) we shall get a single equation between x and y which will be the equation of the curve in one of the forms (1). 138 THE DIPPERENTIAL CALCULUS. Example 1. Let us put a, 5 = the co-ordinates of any fixed point 5 of a straight line; a = the angle which the line makes with the axis of z; p = the distance of any point P of the line from the point {a, I). Then we readily see from the figure that the co-ordinates x and y ot P are given by the equations x = a -\- p cos a; y = b -{- p sin a ■X X . - ^ p/ y a b/ v y^ y y 6 o A 1 X y y y Fio. 16. ;} (3) ^'1 (4) which are equations of the straight line in the independent form. Here p is the auxiliary yariable, called u in Eq. 3. By eliminating this quantity we shall have X sin ot — y cos a = a sin a — i cos a, which is the equation of the line in one of its usual forms. Example 3. The equation of a circle may be expressed in the form a; = a + c cos «; y = b -\- csinu; u being the independent variable. By writing (4) in the form X — a=: c cos u, y — h = c sin u, and eliminating u by taking the gum of the squares of the two equa- tions, we have fio. it. PLANE CURVES. 139 the equation of a circle of radius c. Notice the beautiful relation between (3) and (4). They are the same in form: if in (4) we write p for c and a for u, they will be the same equations. Then, by supposing p constant and a variable, we are carried round the point (a, t) at a constant distance p, that is, around a circle. By suppos- ing p variable and a constant we are carried through (a, V) in a constant direction, that is, along a straight line. 7 1 . Infinitesimal Elements of Curves. Let P and P' be two points on a curve, P being supposed fixed, and P' variable. We may then sup- pose P' to approach P as its limit, and in- quire into the limits of any magnitudes associated with the curve. We may also measure the length of an arc of the curve from an initial point G to a terminal point P. Then, supposing G fixed and P variable, PP' may be taken as an increment of the arc. If we put s = arc GP, we shall have As = arc PP'. Axiom. The ratio of an infinitesimal element of a curve to the straight line joining its extremities approaches unity as its limit. We call this proposition an axiom because a reaUy rigorous demonstration does not seem possible. Its truth will appear by considering that if the curve has no sharp turns, which we presuppose, then it can change its direction only by an in- finitesimal quantity in any infinitesimal portion of its length. Now, a line which has the same direction throughout its length is a straight line. 140 TEE DIFFERENTIAL GALCULU8. 78. Theokem I. If a straight line touch a curve at the point P, a point P' on the curve at an infinitesimal distance will, in general, he distant from the tangent iy an infinitesimal of the second order. Let y = f {x) be the _o . equation of the curve. Leb us transform the equation to a new system of co-ordi- nates, x' and y', so taken that the axis of JT shall be parallel FiQ. 19. to the tangent at P dv' This will make j-, = 0. Let x' and y' be the co-ordinates of P, and {x' -f h, y") the co-ordinates of a point P' near P. Developing by Taylor's theorem, we have y -' dx' ^ dx' 1-2 + .. Now, y" — y'is the distance P'Q of the point P' from the tangent at P. Since dl dx' 0, when h becomes infinitesimal dy h' the term of highest order in this distance is t^^ ^Hy, a quan- tity of the second order. Eemabe. In the special case when d^y' dx" ' : 0, the distance in question may be a quantity of the third or of some higher order, according to the order of the first differential coeffi- cient which does not vanish. CoEOLLAET. The cosine of an infinitesimal arc differs from unity hy an infinitesimal of the second order. For if we draw a unit circle with its tangent at the initial point, the cosine of an arc will differ from unity by the dis- tance from the end of the arc to the tangent line. When the arc is infinitesimal, the coroUary follows from the theorem. PLANE 0UBYE8. 141 Thboeem II. The area included between an infinitesimal arc and its chord is not greater than an infinitesimal of the third order. From Th. I. we may readily see that the maximum distance between the chord and its arc is a quantity of the second order. The area is less than the product of this distance by the length of the chord, which product is an infinitesimal of at least the third order. TO. Expressions for Elements of Curves. Def. An element of a geometric magnitude is an infinitesimal por- tion of that magnitude. The word implies that we conceive the magnitude to be made up of infinitesimal parts. Element of an Arc. Let us put s = the length of any arc of a curve; ds = an element of this arc. If P and P' be two points of a curve, we shall have (chord PPy = ^x' + Ay\ When PP' becomes infinitesimal, as the ratio of ds to PP' becomes unity (§ 77), and we have y^ Aa; ds'' = dx^ + dy''; fio- 20- ds = Vdx' + df = yi + (^'dx. Case of Polar Co-ordinates. To express the element of a curve referred to polar co-ordinates, difEerentiate the equa- tions x = r cos 6; y — r sin 0. Thus dx = cos ddr — r sin 6d0; dy = sin ddr -\- r cos Odd; which gives ds'' = dr^ -{- r^dfP and ds = |/r'+Q'^i9. 142 THE DIFFERENTIAL CALCULUS. 80. Equations of certain Noteworthy Curves. The Cycloid, The cycloid is a curve described by a point on tbe circumfer- ence of a circle rolling on a straight line. A point on the circumference of a carriage-wheel, as the carriage moyes, describes a series of cycloids, one for each revolution of the virheel. To find the equation of the cycloid, let P be the generating point. Let us take the line on which the circle rolls as the axis of X, and let us place the origin at the point where P is in contact with the line OX. FlO. 21. Also put a = the radius of the circle ; u = the angle through which the circle has rolled, expressed in terms of unit-radius. Then, when the circle has rolled through any distance OR, this distance will be equal to the length of the arc PR of the circle between P and the point of contact R, that is, to au. We thus have, for the co-ordinates of the centre, C, at the circle, X = au; y = a; and for the co-ordinates of the point P on the cycloid. a; = «M — a sin M = a{u — sin u); y = a — a cos M = «(! — cos u) ;} (1) which are the equations of the cycloid with w as an independ- ent variable. PLANE CURVES. 143 To eliminate u, find its value from the second equation, u = cos(-« [l - ^\ This gives sin u=: Vl — cos' u = —. ° a Then, by substituting in the first equation .a — : x = a cos<-» ^ - V2ay-y', (3) Cb which is the equation of the cycloid in the usual form. 81. The Lemniscate is the locus of a point, the product of whose distances from two fixed points (called foci) is equal to the square of half the distance between the foci. Let us take the line joining the foci as the axis of X, and the middle point of the segment between the foci as the origin. Let us also put c = half the distance between the foci. Fio. 23. Then the distances of any point {x, y) of the curve from the foci are V{x - cf + y^ and V{x + c)' + y\ Equating the product of these distances to c', squaring and reducing, we find {x^ + yy^2c^{x^-f), (3) which is the equation of the lemniscate. 144 THE DIFFERENTIAL CALOULUS. Transforming to polar co-ordinates by the substitutions a; = r cos d, y ^^r sin B, we find, for the polar equation of the lemniscate, r' = 3c' cos %e. (4) Putting y = 0, we find, for the point in which the curve cuts the line joining the foci, a; = ± ^c = a. The line a is the semi-axis of the lemniscate. Substitut- ing it instead of c, the rectangular and polar equations of the curve will become (x' + fy = a'ix'-y');l r" = a' cos 2(9. ) ^ ' 83. The Archimedean Spiral, This curve is generated by the uniform motion of a point along a line revolving uni- formly about a fixed point. To find its polar equation, let us take the fixed point as the pole, and the position of the revolving line when the generat- ing point leaves the pole as the axis of reference. Let us also put a = the distance by which the generating point moves along the radius vector while the latter is turning through the unit radius. Then, when the ra- dius vector has turned through the angle 6, the point will have moved from the pole through the distance aff. r = a0 as the polar equation of the Archimedean spiral. Fig. 23. Hence we shall have PLANE CURVES. 145 If we increase by an entire revolution (2;r), the corre- sponding increment of r will be 27ra, a constant. Hence: The Archimedean spiral cuts any fixed position of the ra- dius vector in an indefinite series of equidistant points. 83. The Logarithmic Spiral. This is a spiral in which the logarithm of the radius vector is proportional to the angle through which the radius vector has moved from an initial position. Hence, if we put 6^ for the initial angle, we have log r = i{e- ex I being a constant. Hence r = e"'-"' Putting, for brevity, a — e °, the equation of the logarith- mic spiral becomes r = a and I being constants. Fio. 24. EXERCISES. 1. Show (1) that the maximum ordinate of the lemniscate is ic, and (2) that the circle whose diameter is the line join- ing the foci cuts the lemniscate at the points whose ordinates are a maximum. 2. Find the following expression for the square of the dis- tance of a point of a cycloid from the starting point ( 0, Pig. - 21): r' = 2ay + 2uax — a'u'. 3. A wheel makes one revolution a second around a fixed axis, and an insect on one of the spokes crawls from the cen- tre toward the circumference at the rate of one inch a second. Find the equation of the spiral along which he is carried. 146 TEM DIFFEBENTIAL CALCULUS. 4. If, in that logarithmic spiral for which a = 1 and 1 = 1, r = e*, the radius vector turns through an arc equal to log 2, its length will be doubled. 5. If, in any logarithmic spiral, one radius vector bisects the angle between two others, show that it is a mean propor- tional between them. 6. Show that the pair of equations z = au*, y = lu, represent a parabola whose parameter is — . 7. If, in the equation of the Archimedean spiral, 6 and therefore r take all negative values, show that we shall have another Archimedean spiral intersecting the spiral given by positive values of ^ in a series of points lying on a line at right angles to the initial position of the revolving line. This should be done in two ways. Firstly, by drawing the continua- tion of the spiral when, by a negative rotation of the revolving line, the generating point passes through the pole. It will then be seen that the combination of the two spirals is symmetrical with respect to the vertical axis. Secondly, by expressing the rectangular co-ordinates of a point of the spiral in terms of 8 we have x — aB cosB, y =: 06 sin 8. Changing the sign of 8 in this equation will change the sign of x and leave y unchanged. 8. Show that if we draw two lines through the centre of a lemniscate making angles of 45° with the axes, no point of the curve will be contained between these lines and the axis of Y. TANGENTS AND NORMALS. 147 CHAPTER XII. TANGENTS AND NORMALS. 84. A tangent to a curve is a straight line through two coincident points of the curve. Fio. 25. A normal is a straight line through a point of the curve perpendicular to the tangent at that point. The subtangent is the projection, TQ, upon the axis of X, of that segment TP of the tangent contained between the point of contact and the axis of X, The subnormal is the corresponding projection, QN, of the segment PNoi the normal. Notice that a tangent and a normal are lines of indefinite length, while the subtangent and subnormal are segments of the axis of abscissas. Hence the former are determined by their equations, which will be of the first degree in x and y, while the latter are determined by algebraic expressions for their length. But the segments TP and PN are sometimes taken as lengths of the tangent and normal respectively, when we con- sider these lines as segments. 148 THE DIFFERENTIAL OALOULUS. 85. General Equation for a Tangent. The general prob- lem of tangents to a curve may be stated thns: To find the condition which the parameters of a straight line must satisfy in order that the line may be tangent to a given curve. But it is commonly considered in tbe more restricted form: To find the equation of a tangent to a curve at a given point on the curve. Let {x^, «/,) be the given point on the curve. By Analytic Geometry the equation of any straight line through this point may be expressed in the form y-y,=m{x- x,); (5) m being the tangent of the angle which the line makes with the axis of X. But we have shown (§ 20) that m = -f-', aa;, this differential coefficient being formed by differentiating the equation of the curve. Hence y-&. = ^(^-^.) (6) is the equation of the tangent to any curve at a point (a;,, y^j on the curve. Equation of the Normal. The normal at the point (a;,, y,) passes through this point, and is perpendicular to the tangent. If m' be its slope, the condition that it shall be perpendicular to the tangent is (An. Geom.) m'=- — =- — m dy^ dx^ Hence the equation of the normal at the point (a;,, y,) is dx, {y - y.) ^00,-x. (7) TANGENTS AND NORMALS. 149 In these equations of the tangent and normal it is necessary to distinguish between the cases in which the symbols x and y represent the co-ordinates of points on the tangent or nor- mal line, and those where they represent the given point of the curve. Where both enter into the same equation, one set, that pertaining to the curve, must be marked by suflBxes or accents. 86. Suhtangent and Subnormal. To find the length of the subtangent and subnormal, we have to find the abscissa a;, of the point T in which the tangent cuts the axis of abscis- sas. We then have, by definition. Fig. 26. Subtangent = x^ — z„ The value of «„ is found by putting y = and a; = a;, in the equation of the tangent. Thus, (6) gives y. dx. K - «,)• Hence, for the length of the subtangent TQ, Subtangent = a;, — a;„ = -1^. dx^ We find in the same way from (7), for QN, Subnormal = — y,-^. ^'dx. (8) (9) 150 THE DIFWEBENTLAL CALCULUS. 87. Modified Forms of the Equation. In the preceding discussion it is assumed that the equation of the curre is given in the form y =/(«=)• But, firstly, it may be given in the form n^, y) = 0. We shall then have (§37) dF dx. dx^ ' dF' dy. Substituting this value in the equations (6) and 1 Tangent dF, yd = a^i?=. - -x); -^ , dF, . dF, . Normal: —(y-y^) = —{x-x,). (10) Secondly, if the curve is defined by two equations of the form X = 0.(m), X = <p,{u), I y = 0,(m), ) (11) dy^ , dy. du we bave -^ = -rr-, du in which there is no need of suflBxes to x and y in the second member, because this member is a function of u, which does not contain x or y. By substitution in (6) and (7), we find Eq. of tangent: {y - y,)-£- = (« - a;,)^. Eq. of normal: (y - y.)^ = {x, - x)^. (12) TANGENTS AND NORMALS. 151 By substituting in these equations for x^y^, t— and -^ their values in terms of u, the parameters of the lines will be functions of u. Then, for each value we assign to u, (11) will give the co-ordinates of a point on the curve, and (13) will determine the tangent and normal at that point. 88. Tangents and Normals to the Conic Sections. Writing the equation of the ellipse in the form oy + 5V = a'l\ (a) we readily find, by differentiation, dy _ Vx dx ~ a'y' Applying the suflSx to x and y, to show that they represent co-ordinates of points on the ellipse, substituting in (6) and (7), and noting that k, and y, satisfy (a), we readily find: For the tangent: ^ -f ^ = 1. For the normal: —x « = o' — J'. Taking the equation of the hyperbola, - aY + JV = a'd% we find, in the same way. For the tangent: ^ _ M = i. a a' J' For the normal: —x-\ — w = a'-}-J*. Taking the equation of the parabola, y' = 2pa;, we find, by a similar process. For the tangent: y,y = p{x -\- a;,). For the normal: y -~ yi= — -(», — x). 152 THE DIFFEBBNTIAL CALCULUS. 89. Problem. To find the length of the perpendicular dropped from tUe origin upon a tangent or normal. It is shown in Analytic Geometry that if the equation of a straight line be reduced to the form Ax+By+ 0=0, the perpendicular upon the line from the origin is G p = VA' + 5" It must be noted that in the above form the symbol rep- resents the sum of all the terms of the equation of the line which do not contain either x or y. If we have the equation of the line in the form y-y^ = m{x- a;,), we write it mx — y — ma;, -f y^ = 0, and then we have A = m; B=-l; G=y, -ma;,. Thus, the expression for the perpendicular is y, - mx, Vm' + i Substituting for m the values already found for the tan- gent and normal respectively, we find. For the perpendicular on the tangent: </^ + (|)" For tJie perpendicular on the normal: x+v^^ „ _ ' 'dx, _ x,dx, + y ,dy. ^^^^ir TAJraENTS AND NORMALS. 153 Fig. 37. 90. Tangent and Normal in Polar Oo-ordinates. Pboblem. To find the angle which the tangent at any point makes with the radius vector of that point. Let PP' be a small arc of a curve referred to polar co-ordinates; KP, a small part of the radius vector of the point P (the pole being too far to the left to be shown in the figure); K'P', the same for the point P'. KSR, a parallel to the axis of reference. Drop PQlK'P'. Let SPThe the tangent at P. We also put y = angle KPS ■vrhich the tangent makes with the radius vector. Then let P' approach P as its limit. Then QP' = dr; PQ = rdd; PQ rdd ^^^y^qP^lTr-- We also have 1 1 (1) cos y = •"'+'-■ "'Vi^+^yf dr -d'e'' sin y = cos y tan y — /1''+©T (2) Cor. The angle RSP which the tangent makes with the ^xis of reference is y -\- 6. 164 TBE DIFFERENTIAL CALCULUS. 91. Perpendicular from the Pole upon the Tangent and Normal. When y is the angle between the tangent and the radins vector, we readily find, by geometrical construction, that the perpendicular from the pole upon the tangent and normal are, respectively, p =-r sin y and p =■ r cos y. Substituting for sin y and cos y the values already found, we have. For the perpendicular on tangent : p. For the perpendicular on normal : (3) _ r dr 93. Problem. To find the equation of the tangent and normal at a given point of a curve whose equation is expressed in polar co-ordinates. It is shown in Analytic Geometry that if we put p = the perpendicular dropped from the origin upon a line; a=the angle which this perpendicular makes with the axis of X; the equation of the line may be written X cos a -\- y sin a — p = 0. (1) Now, as just shown, the tangent makes the angle y -\- & with the axis of X, and the perpendicular dropped upon it makes an angle 90° less than this. Hence we have a = y + - 90°; cos a = sin (y -\- 6) = sin / cos ^ + cos y sin 6; BID. a = — COB {y -{- d) — — cos y cos ff -^ sin y sin d. TANGENTS AND NOBMALS. 155 By substitution in (1), the equation of the tangent becomes a;(sin ^^ cos 6* + cos y sin 6) — y{coB Y cos 6 — sin y sin ^) — ^ = 0, Substituting for cos y, sin y and^ the values already found, this equation of the tangent reduces to [r cos 6 -{--,-„ sin d\x-\-\r sin ^ — -^a cos ^j «/ — r' = 0, (2) r and 6* being the co-ordinates of the point of tangency. In the case of the normal the perpendicular upon it is parallel to the tangent. Therefore, to find the equation of the normal, we must put in (1) a=y-{-e. Substituting this yalue of a, and proceeding as in the Case of the tangent, we find, for the normal, (-72 cos 5 — rsin (9ja;-f frcos 6 -\- --rj,%va. d\y — rjo = 0. (3) Generally these equations will be more conyenient in use if we divide them throughout by r. Thus we have: Equation of the tangent : (''"^ ^ + r ri'''' ^J'' + r"" ^ ~ fS""^ e^y-r = 0. (4) Equation of the normal : /I dr \rdd''°' 0-sm0)x+ (i g sin + cos e)y-% = 0. (5) In using these equations it must be noticed that the co- efiicients of x and y are functions of r and 6, the polar co- ordinates of the point of tangency. When r, and -^^ are da given, this point and the tangent through it are completely determined. 156 THE DIFFERENTIAL CALCULUS. EXERCISES. I. Show that in the case of the Archimedean spiral the general expressions for the perpendiculars from the pole upon the tangent and normal, respectiyely, are a<9' , ad V(i + ff') — ^ ~ V(i + e")' Thence define at what point of the spiral the radius rector makes angles of 45° with the tangent and normal. Find also what limit the perpendicular upon the normal approaches as the folds of the spiral are continued out to infinity. Show also from § 92 that the tangent is perpendicular to the line of reference at every point for which r sin ^ — a cos 61 = 0, . and "hence that, as the folds of the spiral are traced out to infinity, the ordinates of the points of contact of such a tan- gent approach ± a as their limit. 2. Show by Eq. 13 that in the case of the logarithmic spiral the angle which the radius vector makes with the tan- gent is a constant, given by the equation tan y = J- 3. Show from Eq. 13 that if a curve passes through the pole, the tangent at that point coincides with the radius vector, unless -^ = at this point. Thence show that in the lemniscate the tangents at the origin each cut the axes at angles of 45°. 4. Show that the double area of the triangle formed by a tangent to an ellipse and its axes is . Then show that the a^.y. area is a maximum when — ' = ± f-'. a Show also that the area of the triangle formed by a nor- mal and the axes is a maximum for the same point. ASYMPTOTES AND SINGULAB POINTS. 157 CHAPTER XIII. OF ASYMPTOTES, SINGULAR POINTS AND CURVE-TRACING. 93. Asymptotes. An asymptote of a curve is the limit which the tangent approaches when the point of contact re- cedes to infinity. In order that a curve may have a real asymptote, it must extend to infinity, and the perpendicular from the origin upon the tangent must then approach a finite limit. Por the first condition it suffices to show that to an infi- nite value of one co-ordinate corresponds a real value, finite or infinite, of the other. For the second condition it suffices to show that the expres- sion for the perpendicular upon the tangent (§§ 89, 91) ap- proaches a finite limit when one co-ordinate of the point of contact becomes infinite. If, as will generally be most con- venient, the equation of the curve is written in the form F{x, y) = 0, (1) the value (1) of the perpendicular, omitting suffixes, may be reduced to dF OF {Gfy+(f)T \dy If this expression approaches a real finite limit for an infinite value of x or y, the curve has an asymptote. If the curve is referred to polar co-ordinates, we use the expression (3), § 91, for j». If this approaches a real finite limit for an infinite value of r, the curve has an asymptote. 158 THE DIFFERENTIAL CALCULUS. The existence of the asymptote being thus established, its equation may generally be found from the form (10), § 87, which we may write thus: dF , dF ^ + -j-y '■ dx, dF dF (3) by supposing a;, or y, to become infinite. dF dF Commonly the coefficients -r- and t— will themselves be- •' dx^ dy, come infinite with the co-ordinates. We must then divide the whole equation by such powers of a;, and y, that none of the terms shall become infinite. Fio. 26. Then the equation be- 94. Examples of Asymptotes. 1. F{x) = x'+ y'- 3axy = O.(fl) The curve represented by this equation is called the Folium of Descartes. The equation (3) gives in this case, applying suffixes, = < + y* - 2«a;,y, = ax,y,. To make the coefficients of x and y finite for a;, = oo , divide by a;,y,. comes \y, xj ^ W. yj^ ^ ' Let us now find from (a) the limit of y, for a;, = oo . We have X X The second member of this equation will approach zero as a limit, unless y, is an infinite of as high an order as x,', which is impossible, because then the first member of the equation containing y,' would be an infinite of higher order ASYMPTOTES AND SINQULAB POINTS. 159 than the second member, which is absurd. Hence, passing to the limit. lim.^|j (a;, = «) = -!. Then, by substitution in (J), we find, for the asymptote, » + y + « = 0, %. Take next the equation F{x, y) = x' — %x^y — ax^ — a'y = 0. (a) With this equation (3) becomes (3a;.' - 4:X,y, - 2ax,)x - (2x^' + a')y = 3x' - 6x'y^ - %ax* — a'y,. (J) Pio. 89. We notice that the terms of highest order in the second member are three times those of highest order in (a). From (a) we have x'-2x,'y =fla;,' + ay. Substituting in the second member of (J), and dividing by a;,", (i) becomes (^-^l:-l>-K?V=''+^- <*') Solving (a) for y, we find y, ^ g.' - ax, x, 2a;,' + a" an expression which approaches the limit -J when x, = co. Thus, passing to the limit, {b') gives, for the equation of the asymptote, x — %y=a. 160 TOE DIFFERENTIAL CALCULUS. 3. The Witch of Agnesi. This curve is named after the Italian lady who first investigated its properties. Its equation is x'y -\- a'y — a' = 0. (a) The equation of the tangent is 2x^y^x + {x: + a')y = Zx.'y, + a'y, = 3a' - 2a'y,. (&) By solving (a) for x and y respectively we see that a:, may become infinite, but that y, is always positive and less than a. Hence, to make the coefficient of y in {b) finite for a;, = oo , we must divide by x^, which reduces the equation of the asymptote to Hence the axis of x is itself an asymptote. 95. Points of Inflection. A point of inflection is a point where the tangent inter- sects the curve at the point of tangency. It is evident from the figure that in passing along the curve, and con- sidering the slope of the ^'°- *'• tangent at each point, the point of inflection is one at which this slope is a maximum or a minimum. Because we have slope = -rr-, ^ ax the conditions that the slope shall be a maximum or minimum are and -=-^ different from zero. If the first condition is fulfilled, but if -=^, is also zero, we must proceed, as in problems of maxi- ASYMPTOTES AND SINGULAR POINTS. 161 ma atid minima, to find the first derivative in order which does not vanish. If the order of this derivative is even, there IS no point of inflection for -j^ = Oj if odd, there is one. As an example, let it be required to find the points of in- flection of the curve xy' — a'{a — x). Eeducing the equation to the form wefind % = -^y' The condition that this expression shall vanish is ^xy^ = a', which, compared with the equation of the curve, gives, for the co-ordinates of the point of inflection, 3 a X = -ra; y = ± —T=-. 4: ' " 4/3 EXERCISKS. Find the points of inflection of the following curves ; 1. xy = a" log — . Ans. ° a ix=: a{l — cos m); \y = a ( .K = ae'. (y = %ae~\ y = a{nu + sin m). _ (w + l)a. Ans. ' 2/=«(cosC-«(-^) + Vr?^^l\ 162 THE DIFFERENTIAL CALCULUS. ..-/ Fio. 82. 96. Singular Points of Curves. If we conceive an infini- tesimal circle to be drawn round any point of a curve as a centre, then, in general, the curve will cut the circle in two opposite points only, which will be 180° apart. But special points may sometimes be found on a curve where the infinitesimal circle will be cut in some other way than this: perhaps in more or less than two points; perhaps in points not 180° apart. These are called singular points. The principal singular points are the following: BouMe-poinis j at which a . curve intersects itself. Here the curve cuts the infinitesimal circle in four points (Fig. 33). Ousps; where two branches of a curve terminate by touching each other (Pig. 34). Here the infinitesimal circle is cut in two coincident points. Stopping Points; where a curve suddenly ends. Here the infinitesimal circle is cut in Fia. 35. only a single point. ^_^ Isolated Points; from which no curve proceeds, so K_^ that the infinitesimal circle is not cut at all, fio. 36. Salient Points; from which proceed two branches making with each other an angle which is neither zero nor 180°. Here the infinitesimal circle is cut in two points which are neither apposite nor coincident. There may also be multiple-points, through which the curve passes any number of times. A double-point is a special kind of multiple-point. A multiple-point through which the curve passes three times is called a triple-point. Fis. 33. Fig. 34. ASTMPTOms AND SINGULAB POINTS. 163 X >- / Xo I Q^ ; o / ! / 1 s. 97. Condition of Singular Points. Let (a;„, yj be any point on a curve, and let it be required to investigate the question whether this point is a singular one. We first trans- form the equation of the curve to one in polar co- ordinates having the point (a;„, y^ as the pole. To do this we put, in the equation of the curve, x = x„-\- pcosB;) ^jj y = y. + P sin ft ) The resulting equation between p and d will be the Fia. 37. equation of the curve referred to (a;,, y„) as the pole. More- over, if we assign to /o a fixed value, the corresponding value of 6 derived from the equation will be the angle 6 showing the direction QP from Q to the point P, where the circle of radius p cuts the curve. The limit which 6 approaches as p becomes infinitesimal will determine the points of intersection of the 'infinitesimal circle with the curve. If, now, the given equation of the curve is F{x, y) = 0, then, by the substitution (1), the polar equation vdll be F(x, + p cos e,y,+p sin 6) = 0. (2) Now, let us develop this expression in powers of p by Mac- laurin's theorem. Since p enters into (2) only through x and y in (1), we have dF dFdx , dFdy JtF , . JLF _, dp dx dp dy dp dx dy (because -^- = cos 6 and -r- = sin ^j. Then 164 TEE DIFFERENTIAL CALCULUS. d'F dF' JO'Fdx , d'F dy\ ■i-r = —r- = cos » T-r ^- + 3— j- -4- dp dp \dx dp dxdy dpi 4. sin ei— ^^ d'Fdy\ [dxdy dp dy' dp) = cos" 0-j-,- + 2 sm ^ cos 6-r^r- + sm ^-r-,- = -^ • Noting that when p = then a; = x„, we see that the de- velopment by Maclaurin's theorem will be F{x, y) = Fix,, y,) + p[cos ^g + sin ^g) + etc. = 0. dF dW Here -5— means the value of -5— when x„ is put for a;, etc. Because (a;„, y„) is by hypothesis a point on the curve, we have F{x,, y,) = 0, and the only terms of the second member are those in p, p', etc. Thus the polar equation (3) of the curve may be written F/p + F/'p' + F/"p' + etc. = 0, ) or F/ + F,"p + i?;"'p' + etc. = 0. f ^"^ To find the points in which the curve cuts a circle of radius p, we have to determine ^ as a function of p from this equa- tion. When p is an infinitesimal, all the terms after the first will be infinitesimals. Hence, at the limit, where p becomes infinitesimal B must satisfy the equation dF which gives tan 6 = — — ~. ^. This is the known equation for the slope of the tangent at (a;„, yj, and gives only the evident result that in general the ASYMPTOTES AND SINGULAR POINTS. 165 curve cuts the infinitesimal circle along the line tangent to the curve at Q. But, if possible, let the point (a;„y„) be so taken that J^ = 0; §^ = 0. (4) dx, ' dy„ Then we shall have FJ = 0, and the equation (3) of the curve will reduce to F:'P + F:"p' + etc. = 0, or F," + F,"'p + etc. = 0. Again, letting p become infinitesimal, we shall have at the limit Dividing throughout by cos' 6, we shall have a quadratic equation in tan 6, which will have two roots. Since each value of tan 6 gives a pair of opposite points in which the curve may cut the infinitesimal circle, and since (5) depends on (4), we conclude: The necessary condition of a double-2)oint is that the three equations shall be satisfied by a single jmir of values of x and y. If the two values of tan derived from FJ' = are equal, we shall have either a cusp, or a point in which two branches of the curve touch each other. If the roots are imaginary, the singular point will be an isolated point. 98. Examples of Double-points. A curve whose equation contains no terms of less than the second degree in x and y has a singular point at the origin. Eor example, if the equa- tion be of the form F(x, y) = Px' + Qxy + By' = 0, then this expression and its derivatives with respect to x and y will vanish for x = Q and y = 0. 166 TEE DIFFERENTIAL OALCULm. Let us now investigate the double-points of the curve (y' - a'Y - 3aV - 2ax' = 0. (1) We have dF dx dF dy = -6(a'ar4-aa;') Qax{a -f x); = ^y{y^ - a') = 4«/(y + «) (y - a). (3) The first of these derivatives vanishes for a; = or — a; The second of these derivatives vanishes f or y = 0, — a or + a. Of these values the original equation is satisfied by the fol- lowing pairs: a;„= 0; 0; -a;) y,= -a; + a; 0;) ^^^ which are therefore the co-ordinates of singular points. FlO. 38. Differentiating again, we have Forming the equation F" = 0, it gives ASYMPTOTES AND SINGULAR POINTS. 167 (12^' - 4a'') tan' (? = 6a' + nax. Substituting the pairs of co-ordinates (3), we find: At the point (0, — a), ian.e= ±^V2; At the point (0, + a), tan l9 = ± ^ V3; At the point (— a, 0), ia.n 6 — ± l/f. The Talues of tan 6 being all real and unequal, all of these points are double-points. The curve is shown in the figure. Eemabk. In the preceding theory of singular points it is assumed that the expression (2), § 97, can be developed in powers of p. If the function F is such that this development is impossible for certain values of x^ and y„, this impossibility may indicate a singular point at (a:,, y^. 99. Curve-tracing. We have given rough figures of va- rious curves in the preceding theory, and it is desirable that the student should know how to trace curves when their equations are given. The most elementary method is that of solving the equation for one co-ordinate, and then substitut- ing various assumed values of the other co-ordinate in the solution, thus fixing various points of the curve. But un- less the solution can be found by an equation of the first or second degree, this method will be tedious or impracticable. It may, however, commonly be simplified. 1. If the equation has no constant term, we may sometimes find the intersections of the curve with a number of lines through the origin. To do this we put y = mx in the equation, and then solve for x. The resulting values of a; as a function of m are the abscissas of the points in which the curve cuts the line y — mx = 0. Then, by putting m= ±1, ±2, etc.; in = ± i, ± \, etc., we find as many points of intersection as we please. 168 THE DIFFERENTIAL GALCULU8. To make this method practicable, the equations which we have to solve should not be of a degree higher than the second. If the curve has a double-point, it may be convenient to take this point as the origin. 2. If the equation is symmetrical in z and y ot x and — y, the curve will be symmetrical with respect to one of the lines X — y ^=Q and a; + y = 0. The equation may then be simplified by referring it to new axes making an angle of 45° with the original ones. The equations for transforming to such axes are x={x' -\- y') sin 45°; y — {x' — y') sin 45°. Application to the Folium of Descartes. If, in the equa- tion of this curve, x' -\-y' = 3axy, we put y = mx, we shall find Sam 3am' ^ — 1 _L ^y> y — ■ l + w" ^ l+m=' We also find, from the equation of the curve and the pre- ceding expressions for x and y in terms of m, dy _x' — ay _2m — m* dx ax -y'~ 1 - 2m'' Then, for m = 1, 3 y = 3 2"' ^y -^ 1 dz m — 3, 2 y = 4 3^; dy 4 dz 5" m = 3 2' 36 " = 35^' y = 54 dy 33 dz ~ 92' m= — ■2, 6 x = ^a; y = - 12 dy 20 dx ~ 17" etc. etc. etc , etc. Thus we have, not only the points of the curve, but the tangents of the angle of direction of the curve at each point, which will assist us in tracing it. THEORY OF ENVELOPES. 169 CHAPTER XIV. THEORY OF ENVELOPES. 100. The equation of a curve generally contains one or more constants, sometimes called parameters. For example, the equation of a circle, {x - ay + {y- by = r\ contains three parameters, a, h and r. As another example, we know that the equation of a straight line contains two independent parameters. Conceive now that the equation of any line, straight or curve, (which we shall call "the line" simply,) to be written in the implicit form ^{«, y, «) = 0, (1) a being a parameter. By assigning to a the several values a, a', a", etc., we shall have an equal number of lines whose equations will be 0(«, y, oi) = 0; (t>{x, ij, a') = 0; ^{x, y, a") = 0; etc. The collection of lines that can thus be formed by assign- ing all values to a parameter is called a family of lines. Any two lines of the family, e.g., those which have a and a' as parameters, will in general have one or more points of intersection, determined by solving the corresponding equa- tions for X and y. The co-ordinates, x and y, of the point of intersection will then come out as functions of a and a'. Suppose the two parameters to approach infinitesimally near each other. The point of intersection will then approach a certain limit, which we investigate as follows; 170 THB DIFFERENTIAL CALCULUS. Let us put a' ■= a-\- Aa, The equations of the lines will then be <p{x, y, a) =0 and ^(x, y, a-\- /la) = 0. If we develop the left-hand member of the second equation in powers ot Aahj Taylor's theorem, it will become <t>{x, y, a) + -^Aa + ^ 3-3 + etc. = 0. Subtracting the first equation, dividing the remainder by Aa, and passing to the limit, we find d(l>{x, y, a) da 0. Hence the limit toward which the point {x, y) of intersec- tion of two lines of a family approaches as the difference of the parameters becomes infinitesimal is found by determining z and y from the equations <Pix,y,a)^0 and M^^) = 0. (2) The values of x and y thus determined will, in general, be functions of a; 'that is, we shall have a= =/.(«); 2/ =/.(«); (3) which will give the values of the co-ordinates x and y of the limiting pojnt of intersection for each value of a, Jfow, suppose a to vary. Then x and y jn (3) will also vary, and will determine a curve as the locus of x and y. Such a curve is called the envelope of the family of lines, (j){x, y, a) = 0, In (3) the equations of the curve are in the form of (2), § 76, a being the auxiliary variable. By eliminating a either from (2) or (3), we have an equation between x and y which wiU be the equation of the curve in the usual form. THEORY OF ENVELOPES. Ill lOl. Theorem. The envelope and all the lines of the family which generate it are tangent to each other. Oeometrically the truth of this will be seen by drawing a series of lines varying their position according to any con- tinuous law, as in the first example of the following sec- tion. Taking three consecutive lines and numbering them (1), (3) and (3), it will be seen that as (1) and (3) approach (2) their points of intersection with (3) approach infinitely near each other. Since these infinitely near points of inter- section also belong to the envelope, the line (3) passes through two infinitely near points of the envelope and is therefore a tangent to the envelope. Analytic Proof. The equation of the envelope is found by eliminating a from the equations (3), and we may conceive this elimination to be efEected by fin.ding the value of a from the second of these equations (3), and substituting it in the first equation. That is, the equation <j){x, y, a) = (4) represents any line of the original family when we regard a as a constant; and it represents the envelope when we regard a as a function of x and y, satisfying the equation d<p{x y,a)^^^ da ^ ' Let the value of a derived from this last equation be a = F{x, y). (6) Now, to find the slope of the tangent to the original line of the family at the point {x, y), we difEerentiate (4), regardiug « as a constant. Thus we have ^ I ^^ 1^ = or ^ = - ^ dx dy dx dx Dy<p' ^ ' If the original line is a straight one, this equation will give its slope. To find the slope of the tangent to the envelope at the saffie 172 THE DIFFERENTIAL CALCULUS. point, we diflEerentiate this same equation, regarding a as hav- ing the value (6), Thus we have d<p d<p dy Ax dy dx d(p d^fda da dy\ _ + d^w + ^ ^y ~ ^^' But, because -p- = 0, this equation will also give the value (7) for the slope; whence the curves have the same tangent at the point {x, y), and so are tangent to each other at this point. 103. We shall now illustrate this theory by some examples. 1. To find the envelope of a straight line which moves so that the area of the triangle which it forms with the axes of co- ordinates is a constant. Fig. 39. Since the area of the triangle is half the product of the intercepts of the axes cut off by the line, this product is also constant. Calling a and 5 the intercepts, the equation of the line may be written in the form 0(a;, 2/, a) = I + 1 - 1 = 0. (1) THEORY OF ENVELOPES. 173 Here we haye two varying parameters, a and h, while, to have an envelope, the change of the parameters must depend on a single varying quantity. But the condition that the product of the intercepts shall be constant enables us to elimi- nate one of theparameters, say i. We have, by this condition, h = -, (2) , dh c whence t— = .. da a Now differentiating the equation (1) with respect to a, re- garding 5 as a function of a, we have d<j) _ X y dh _cy — Vx _y x c a' da~ a' b' da ~ a'b' ~ - -' ~ ^' ^^^ We have now to eliminate a from the equations (1) and (3), using (3) to eliminate b from (1). The easiest way to effect this elimination is as follows: From (3) we have I ex a'y = cx; a = W--. (4) Multiplying (1) by a, and substituting for b its value from (3), we have xA — a. c Substituting from (4), this equation becomes V y and thus the equation of the envelope becomes xy = Ic which is that of an hyperbola referred to its asymptotes. This result coincides with one already found in Analytic Geometry, that tangents to an hyperbola cut ofp from the asymptotes intercepts whose product is a constant. 174 THE niFFEBENTIAL CALCULVS. 2. To find the envelope of the line for which the sum of the intercepts cut off from the co-ordinate axes is a constant. Fio. 40. Let c be the constant sum of the intercepts. Then, if a be the one intercept, the other will be c — a. Thus the equa- tion of the line is X a -^ = 1, c — a in ■which a is the varying parameter. Clearing of fractions, we may write the equation <p{x, y, a) = ex -\- a{y — x — c) -\- a' = 0, d^ da whence -~ = y -x-c-^-ia-O. From the last equation we haye a-\{x-y-^ c); this value of a being substituted in the other gives cx — l{x-y-\- cf = 0, or (x — y)' — 2c(x + y) + c' = 0. THEOBT OF ENVELOPES. 175 This equation, being of the second degree in the co-ordi- nates, is a conic section. The terms of the second degree forming a perfect square, it is a parabola. The equation of the axis of the parabola is a; — y = 0. To find the two points in which the parabola cuts the axis of X we put y = 0, and find the correBponding values of x. The resulting equation is x' — 2cx + c' = 0. This is an equation with two equal roots, x =^ c, showing that the parabola touches the axis of X at the point (c, 0). It is shown in the same way that the axis of Y is tangent to the parabola. It may also be shown that the directrix and axis of the parabola each pass through the origin, and that the parame- ter is V2c. 3. If the difference of the intercepts cut off by a line from the axes is constant, it may be shown by a similar process that the envelope is still a parabola. This is left as an exer- cise for the student, who should be able to demonstrate the following results : (a) When the sum of the intercepts is a positive constant, the parabola is in the first quadrant ; when a negative con- stant, the parabola is in the third quadrant. {/3) When the difference, a — l, of the intercepts is a posi- tive constant, the parabola is in the fourth quadrant; when a negative constant, in the second. (/) The co-ordinate axes touch the parabola at the ends of the parameter. In each case the parabola touches each co-ordinate axis at a point determined by the value of the corresponding inter- cept when the other intercept vanishes, and each directrix intersects the origin at an angle of 45° with the axis. 176 THE DIFFERENTIAL OALCULUS. 4. Next take the case in wMch the sum of one intercept and a certain fraction or multiple of the other is a constant. Let m be the fraction or multiplier. We then have I -{-ma = c = a. constant. The equation of the line then becomes a c — ma Proceeding as before, we find the equation of the envelope to be {mx — yf — 2c{mx + y) -|- c' = 0, which is still the equation of a parabola. 5. To find the envelope of a line which cuts off intercepts subject to the condition ^+5.=!, («) m and n being constants. We may simplify the work by substituting for the varying intercepts a and b the single variable parameter a determined by either of the equations m n sm a = — ; cos a = 7-. a The equation of the varying line will then oecome Mx, «) = — sin or + — cos a = 1. (1) By differentiating with respect to a, we have T— = — cos a — — sm a = 0. (2) da m n We may now eliminate a by simply taking the sum of the squares of these equations, which gives the equation of an ellipse whose semi-axes are m and n. TEEORT OF ENVELOPES. 177 6. To find the envelope of a circle of constant radius whose centre moves on a fixed circle. For convenience let us take the centre of the fixed circle as the origin, and put: a, b, = the co-ordinates of the centre of the moving circle; c = its radius; d = the radius of the fixed circle. The equation of the moving circle now becomes (z - a)' + {y- ly - c' = 0. (1) By differentiation with respect to a, x-a+{y-i)^ = (). The condition that (a, h) lies on the fixed circle gives a' + J' = <f% (2) , dl a whence t- = — r- da Then, by substituting this value, ay — lx = 0. (3) We have now to eliminate a and h from (1), (2) and (3). Firstly, from (1) and (2), we find a;' + 2^' - %ax - %by = c' - d\ {!') From (2) and (8) we find the following expressions for a and 5: xd , yd a = , ==r; J = ■—===. Va' + f Vx' + y' By substitution in (1'), and putting for brevity r' = x' + y\ we find r' ± 2rd + d' = c'. Hence r' = x' + y' = (c ± df, the equations of two circles around the origin as a centre, with radii c -\- d and c — d. 178 THE DIFFEBENTIAL CALCULUS. 7. Find the envelope of a family of ellipses referred to their centre and axes, the product of whose semi-axes is equal to a certain constant, c". -4ms. The equilateral hyperbola xy = ^c'. 8. To find the envelope of a family of straight lines, such that the product of their distances from two fixed points is a constant. Let (ffl, 0) and (— a, 0) be taken as the two fixed points, and let c^ be the constant. Also, let X cos a -\- y sin a — p ■=<) (1) be the equation of any one of the lines in the normal form, p and a being the varying parameters. The distances of the line from the points (a, 0) and (—a, 0) are respectively — p -\- a cos a and — p — a cos a. Hence we have the condition jo' — a' cos' a = c'. (2) DifEerentiating (1), regarding p asa function of a, we have From (2) we obtain — X sm a 4- y cos a — , " da dp __ a' sin a cos a da~ P ' thus have the three equations a; cos a + y sin « = p. (a) a' sin a cos a xsm a — y cos a = , (S) p' = c' + a' cos' a — (^-\-a* — a* sin' a. (c) from which to eliminate p and a. -r «. y P ' sm a • P X cos a c' + a' P ' y sm a THEORY OF ENVSLOPMS. 179 To effect the elimination of a and p we find the values of K and y from (a) and {b) by taking (a) X cos a + (J) x sin a and (a) X sin a — (S) X cos a. We thus find, by the aid of (c), px = p' cos a-]- a' sin" or cos a; , , , ,,cos a x= {c^ -\- a') Hence c -j- a' K we multiply the first of these equations by x and the second by y.and add, then we have a;' , y' _ * cos a + y sin or _ cM^"? "*" ? ~ ;? ~ Hence the equation of the envelope is This represents an ellipse whose foci are the two fixed points. This interpretation, however, presupposes that the product c' of the distances of the line from the two points is positive; that is, that the points are on the same side of the enveloping line. If the product is negative, the equation of the envelope will be y' _ 1 C c- which is the equation of an hyperbola. These results give the theorem of Analytic Geometry that the product of the distances of a tangent from the foci of a conic is constant. 180 THE DIFFERENTIAL CALCULUS. CHAPTER XV. OF CURVATURE; EVOLUTES AND INVOLUTES. 103. Position; Direction; Curvature. The position of any point P on a curve is fixed by the values of the co-ordi- nates, X and y, of P. This is shown in Analytic Geometry. If we have given, not only x and y, but the value of -^ for the point P, then such value of the derivative indicates the direction of the curve at the point P, this direction being the same as that of the tangent at P, The curve may also have a greater or less degree of curvo' ture at P. The curvature is indicated by a change in the di- rection of the tangent, that is, in the value of —-, when we pass to an adjacent point P', But such change in the value of ■— when we vary x is expressed by the value of the second derivative -r^. If this quantity is positive, the angle which the tangent makes with the axis of ^is increasing with x at the point P, and the curve, viewed from below, is convex. d'ti If -^ is negative, the tangent is diminishing, and the curve, seen from below, is concave. To sum up: If we take a value of the abscissa x, then the corresponding value of y gives the position of a point P of the carve; -^ gives the direction of the curve at P; -^ depends upon the curvature of the curve at P. CURVATURE; E VOLUTES AND INVOLUTES. 181 104. Contacts of Different Orders. Let two different curves be given by their respective equations: y =f{^) aiid y = (j>{x). If for a certain value of x, whicli value call a;„, the two values of y are equal, the two curves have the corresponding point in common; that is, they meet at the point (a;„, y). If the values of ~ are also equal at this point, it shows that the curves have the same direction at the point of meet- ing. They are then said to touch each other. d'v If the values of -=-^ are also equal at this point, the two curves have also the same curvature at this point. To show the result of these several equalities, let us give the abscissa a;„ (which we still take the same for both curves), an increment h, and develop the two values of y in powers of h by Taylor's theorem. To distinguish the values of y, -r-, etc., which belong to the two curves, we assign to one the suflSx 0, and to the other the suffix 1. Then, for the one curve, and, for the other, '■-.+(l),^+(S^),^.+--+(g)|+- The difference between the values of y' and y is the inter- cept, between the two curves, of the ordinate at the point whose abscissa is a;„ + ^- Its expression is ¥ y.-y.+ [diJ, \di)j' + \\dx')~ tv 1.2+ etc. Now, consider the case in which the curves meet at the point P, whose abscissa is a„. Then 182 THE DIFFERENTIAL CALCULUS. and the intercept of the ordinate will be which, when Ji becomes infinitesimal, is an infinitesimal of the first order. If we also have the ordinates will differ only by a quantity containing A* as a factor, and so of the second order. Hence: Wlien two curves are tangent to each other, they are sepa- rated only by quantities of at least the second order at an in- finitesimal distance from the point oftangency. In the same way it is shown that if the second differential coefficient also yanishes, the separation will be of the third order, and so on. Def. When two curves are tangent to each other, if the first n differential coefficients for the two curves are equal at the point of tangency, the curves are said to have contact of the nth order. Hence a case of simple tangency is a contact of the first order. If the second derivatives are also equal, the contact is of the second order, and so forth. 105. Theorem. In contacts of an even order the two curves intersect at the point of contact ; in those of an odd order they do not. For, in contact of the nth. order, the first term oiy' —y (§ 104) which does not vanish contains A"+' as a factor. If n is odd, n-\-l is even, and y' — y has the same alge- braic sign whether we take h positively or negatively. Hence the curves do not intersect. If n is even, m + 1 is odd, and the values oi y' — y have opposite signs on the two sides of the point of contact, thug showing that the curves intersect. CURVATURE; EV0LUTE8 AND INVOLUTES. 183 106. Radius of Curvature. The curvature at any point is measured thus: We pass from the point P to a point P' in- finitesimally near it. The curvature is then measured by the ratio of the change in the direction of the tan- gent (or normal) to the distance PP'. Let us put a = the angle which the tangent at P makes with the axis of X. a-\- da = the same angle for the tangent at P'. ds = the infinitesimal distance PP'- Then, by definition. Curvature = -r-. ds Flo. 41. Now, because we have, by differentiation. *"^'' = £' Also, and sec' ada = ■-j^,dx. sec' a—l-\- tan" a = 1 + -^, ds = /( ^+%y From these equations we readily derive /7/v Curvature da ds dx' (^ + g)* Now, draw normals to the curve at the points P and P', and let C be their point of intersection. Because they are perpendicular to the tangents, the angle PCP' between them will be da, and if we put 184 THE DIFFERENTIAL CALCULUS. we shall have pp, = ds = ■■ pda. Hence p = ds ~da 1 ~ cuTTature \ ^ dxV d'y • dx' The length p is called the radius of curvature at the point P, and C is called the centre of curvature. CoBOLLAET. The centre of curvature for any point of a curve is the intersection of consecutive normals cut- ting the curve infinitely near that point. pV 10 T. The Osculating Circle. If, on the normal PC to any curve at the point P, we take any point ^^ ^ as the centre of a circle through P, that circle will be tangent to the curve at P; that is, it will, in general, have contact of the first order at P- But there is one such circle which has contact of a higher order, namely, that whose centre is at the centre of curvature. Since this circle will have the same curvature at P as the curve itself has, it will have contact of at least the second order at P. This proposition is rigorously demonstrated by finding that circle which shall have contact of the second order with the curve at the point P. Let us put X, y, the co-ordinates of P; jo = ~- for the curve at the point P; q = -t4 for the curve at the point P, aUBVATUBE; EV0LUTE8 AND INVOLUTES. 185 These last two quantities are found by difEerentiating the equation of the curre. Now, -^ and -^4 must have these same values at the point dx d^ (x, y) in the case of the circle having contact of the second order (§ 104). Let the equation of this circle be {X - ay + (y- iy = r\ («) By differentiation, we have {x — a)dx -\- {y — h)dy = 0, , dy X — a ,,. ^^"°°" d^'^l^y^P- <*) DifEerentiating again. ^ - _i_ j_ (^ -a)dy _ {y - by + (r - ay dx' b-y'^ ip-yy dx (y - dy r ~{y- by ^' From (S) combined with (a) we find {x-ay_ {») l+i»= = l + {y - by (y - by (i+/r- r (y - by Dividing this by (c) gives q ' the equivalent of the expression already found for the radius of curvature. Hence if we determine a circle by the condition that it shall have contact of the second order with the curve at the point P, its radius will be equal to the radius of curvature. This circle is called the osculating circle for the point P. Each point of a curve has its osculating circle, determined by the position, direction and curvature at that point. 186 THE DIFFERENTIAL CALCULUS. Cor. The osculating circle will, in general, intersect the curve at the point of contact, for it has contact of the second order. This may also be seen by reflecting that the curvature of a curve is, in general, a continuously varying quantity as we pass along the curve, and that, at the point of contact, it is equal to the curvature of the circle. Hence, on one side of the point of contact, the curvature of the curve is less than that of the circle, and so the curve passes without the circle; and on the other side the curvature of the curve is greater, and thus the curve passes within the circle. If, however, the curvature should be a maximum or a minimum at the point of contact, it will either increase on both sides of this point or diminish on both sides, whence the circle will not intersect the curve. 108. Radius of Curvature when the Abscissa is not taken as the Independent Variable. Suppose that, instead of x, some other variable, u, is regarded as the independent vari- able. We then have Now, it has been shown that, in this case, we have (§ 56) d'y dx d'x dy d'y _ du' du du' du /„> \du I Also, we have fdyV fdxy fdyV 1 , l^y\'--i , ^(^'"'' _\du) '^\dul .„, ^ + WJ - ^ + Jd^' - my — • ^^^ \du I \du I These expressions being substituted in the expression for the radius of curvature, it becomes CUBVATUBE; EVOLUTES AND INVOLUTES. 187 ( \du) + l^j [ ,,, (4) cPy dx d'x dy du' du du' du 109. Radius of Curvature of a Curve referred to Polar Co-ordinates. Let the equation of the curve be given in the form r = (l){d). The preceding expression (4) may be employed in this case by taking the angle 6 as the independent variable. By differ- entiating the expressions X = r cos 6, y = r sin 6, regarding r as a function of 6, we find, when we put, for brevity, , dr ,, d'r r ^ — r zrz -dd' dd-" — = — r sm d -\-r' cos 6; d'x -^ = (r" — r) cos e — 2r' sin 6; ^—r cos -{- r' sin 6; ^ = (r" - r) sin 6 + 2r' cos 6. By substituting these derivatives with respect to 6'for those with respect to u in (4) and performing easy reductions, we find _ (r' + r'y _ \ ^ K ddl f which is the required expression for the radius of curvature. 188 THE DIFFERENTIAL CALCULUB EXAMPLKS AND EXERCISES. 1. The Parabola. To find the radius of curvature of a curve at any point, we have to form the value of p from the equation of the curve. The equation of the parabola is y' = 2px, whence we find dx y' ^ - -t dx' ~ y'' Then, by substituting in the expression for p, we find p- f ' the negative sign being omitted, because we have no occasion to apply any sign to p. At the vertex y = 0, whence p=p. Hence, at the vertex, the radius of curvature is equal to the semi-parameter, and the centre of curvature is therefore twice as far from the vertex as the focus is. 2. Show that the radius of curvature at any point {x, y) of an ellipse is a*b* P = „^7.4 and show that at the extremities of the axes it is a third pro- portional to the semi-axes. 3. Show that the algebraic expression for p is the same in the case of the hyperbola as in that of the ellipse. 4. What must be the eccentricity of an ellipse that the cen- tre of curvature for a point at one end of the minor axis may lie on the other end of that axis? Ans. e = ^/J. dx -^— = a — a coBu du = y; d'x dy -5-, =-r- = asiau; du' du d'y — = a cos u. OURVATUBE; EV0LUTE8 AND INVOLUTES. 189 5. Show that in the case supposed in the last problem the radius of curvature at an end of the major axis will be one fourth that axis. 6. The Cycloid. By differentiating the equations (1), § 80, of the cycloid, we find (3) Then, by substituting in (4) and reducing, we find, for the radius of curvature, p ■= 2*a Vl — cos w = 4a! sin \u. "We see that at the cusp, 0, of the cycloid, where m = 0, the radius of curvature also becomes zero. 7. The Archimedean Spiral. Show from (5) that the ra- dius of curvature of this spiral (r = aO) is 8. The Logarithmic Spiral. The equation of the loga- rithmic spiral being w r = ae , show that the radius of curvature is p = r vr+f. Hence show that the line drawn from the centre of curva- ture of any point P of the spiral to the pole is perpendicular to the radius vector of the point P- 9. Show that the radius of curvature of the lemniscate in terms of polar co-ordinates is a a' P = 3 Vcos %e 3r- 190 THE DUVEBENTIAL CALCXTLUS. no. Evolutes and Involutes. For every point of a curve there is a centre of curvature, found by the preceding for- mulae. The locuB of all such centres is called the evolute of the curve. To find the evolute of a curve, let {x,y^) be the co-ordi- nates of any point P of the curve ; PO, the radius of cur- vature for this point; and a, the angle which the tangent at P makes with the axis of X. Then, for the co-ordinates of 0, we have X =^ Xj — p sm a; y = yj + P0OB a. Substituting for p its value (§ 106), and for sin a and cos a their values from the equation _^ dxf we find Fio. 48. tan a ■■ «> — 1 + ^' dx' dy,^ = 2// 4 dx; ^ dx; dx; dx. (1) If in the second members of these equations we substitute the values of the derivatives obtained from the equation of the curve, we shall have two equations between the four vari- ables X, y, x^ and y^. By eliminating x^ and y from these equations and that of the given curve, we shall have a single equation between x and y, which will be that of the evolute. CUBVATUBBj EVOLUTES AND INVOLUTES. 191 111. Case of an Auxiliary Variaile. If the equation of the curve is expressed by an auxiliary variable, we have to make in (1) the same substitution of the values of -^', -r^./, etc, , as in § 108. Thus we find, instead of (1), fdxX Idy^ dy^ \dul \du du d'y, dx, d'x, dy/ du' du du' du IdxX IdyX dx, \du I \du I which are the equations of the evolute in the same form. du d'y^ dx, d'x, dy^ ' du^ du du' du (3) EXAMPLES OF EVOLUTES. 113. The Evolute of the Parabola. If we substitute in (1) for the derivatives of y, with respect to x, the values already found for the parabola, these equations (1) become , V/' ,3 y; ' -^ p ^ ill p " p We now have to eliminate y^ from these two equations, x^ having already been eliminated by the equation of the curve. They give y/ = M^-p)> y/ = -fy- Equating the cube of the first equation to the square of the second, we find, for the equation of the evolute of the parabola. y = 8 (x - pY 27 P 192 THE BIFPBBBNTIAL GALOULUS. 113. Evolute of the Ellipse. Prom the equation of the ellipse, we find dx^ ai'y/ dx/ o^y'' By substituting in (1) and reducing, we find Eemarking that the equation of the ellipse gives a'V - a*y; = a'5V, and putting e' = a' — S", the preceding equation becomes X = -^. (a) In the same way we get % In this case the easiest way to effect the elimination of x, and y^ is to obtain the values of these quantities from («) and (5), and then substitute them in the equation of the ellipse. Prom (a) and (p), we find which values are to be substituted in the equa- tion 1. ^^ 1- ■^\^ /C.y \ V^B P \ V N / -^ V We thus find, for the equation of the evolute of the ellipse, ffM + b^y^ = c*. The figure shows the ^"^ ^*- form of the curve. The following properties should be de- duced by the student. OU^VATUBE; EVOLUTES AND IN70LUTE8. 193 (a) The evolute lies wholly within the ellipse, or cuts it (as in the figure), according as e' < ^ or e' > \. ifi) The ratio CD : AB (which lines we may call axes of the evolute) is the inverse of the ratio of the corresponding axes of the ellipse. 114. Evolute of the Cycloid. Here we have to apply the formulae (3) for the case of a separate independent variable. Substituting in (2) the values of the derivatives already given for the cycloid, we shall find IdxV \dul d^y dx du' du d'x dy _ cos u); — cos u); du' x= Xi-\-%a sin u = a{u -\- sin u); y ^= y, — 2a(l — cos u) = — a{l — cos u). These last two equations are those of the evolute. Let us investigate its form. For m = we have x = and y — 0, whence the origin is a point of the curve. For u = TT we have ^ Y c Fio. X 45. y=-%a; giving a point C, below the middle of the base of the cycloid, at the dis- tance 2a. Let us take this point as a new origin, and call the co-ordinates referred to it x' and y' . We then have x' ^^ X — an =■ a(d — n -\- sin ff); y' — y -\-%a = a(l + cos ff). If we now put these equations become 194 THE DIFFERENTIAL CALCULUS. x' = a{0' - sin (9'); y' = a{l - cos 6'); which are the equations of another cycloid, equal to the original one, and similarly situated. The cycloid therefore posesses the remarkable property of being identical in form with its own evolute. 115. Fundamental Properties of the Evolute. Theorem I. The involute of a curve is the envelope of its normals. As we moTe along a curve, the normal will be a straight line moving according to a certain law depending upon the form of the curve. This line will, in general, have an en- velope, which envelope will be, by definition, the locus of the point of intersection of consecutive normals. But this point has been shown to be the centre of curvature, whose locus is, by definition, the evolute. p. Hence follows the theorem. OoKOLLAET. The nor- mals to a curve are tan- gents to its evolute. For this has been shown to be true of a moving line and its envelope. Theokem II. If the os- culating circle move around the curve, the motion of its centre is along the line join- ing that centre to the point of contact. This theorem will be made evident by a study of the figure. If the line F^C, be one of the nor- mals from the point of contact P, to the centre, then, since Fio. 46. CUMVATUBB ; EVOLUTES ANB INVOLUTES. 195 this normal is tangent to the locus of the centre, it will be the line along which the centre is moving at the' instant. Theobem III. The arc of the evolute contained between any two points is equal to the difference of the radii of the osculating circles whose centres are at these points. For, if we suppose the points C,, 0^, etc., to approach in- finitesimally near each other, then, since the infinitesimal arcs C^C^, 0,0^, etc., are coincident with those successive radii of the osculating circle which are normal to the curve, these radii are continually diminished by these same infini- tesimal amounts. The analytic proof of Theorems II. and III. is as follows: Let the equation of the osculating circle be {X - ay + {y-~ iy = p\ where a and 5 are the co-ordinates of the centre of curvature, and therefore of a point of the evolute. The complete differential of this equation gives {x — a) {dx — da) -\- (jy — i) {dy — db) = pdp. (a) If, in this equation, we suppose x and y to be the co-ordi- nates of the point of contact of the circle with the curve, then dx and dy will have the same value at this point whether we conceive them to belong to the circle, supposed for the mo- ment to be fixed, or to the curve. But in the fixed circle we have {x — a)dx + (y - b)dy = 0. (5) Subtracting this equation from (a) and dividing by p, we find da + db = — dp, (c) p p which is a relation between the differential of the co-ordi- nates of the centre and the differential of the radius. Now, if we put /3 for the angle which the normal radius makes with the axis of X, we have X — a y — a . = cos yS; = sm p. (d) 196 THE DIFFERENTIAL CALCULUS. But this same normal radius is a tangent to the evolute. If we call <T the arc of the evolute, we find by a simple con- struction da = cos I3d(r; db = sin ftd(T, Multiplying these equations by cos /S and sin C, respectively, and adding, we find d(T = cos /3da + sin /3dl>. Comparing (c) and (d), we find d(T =■ — dp, or d((T + /o) = 0. Now, a quantity whose differential is zero is a constant. Hence we always have a -\- p = constant, or cr = constant — p. If we represent by a-, and o", the arcs from any arbitrary point of the involute to the two chosen points, and by /a, and p, the values of p for these points, we have <r, = const. — p,; <7, = const. — Pj. .'. 0-, - 0-, =P, - A> or the intercepted arc equal to the difference of the radii, as was to be proved. It must be remarked, however, that whenever we pass a cusp on the evolute, we must regard the arc as negative on one side and positive on the other. In the case of the ellipse, for example, those radii will be equal which terminate at equal distances on the two sides of any cusp, as A, B, or D, and the intercepted arc must then be taken as zero. 116. Involutes. The involute of a curve O is that curve which has C as its evolute. The fundamental property of the involute is this: The involute may be formed from the evolute by rolling a tangent OUBVATUBOEj EV0LUTE8 AND INVOLUTES. 197 line upon the latter, A point P on the rolling tangent will then describe the involute. This will be seen by reference to Fig. 46. The rolling line, being tangent to the evolute, coincides with the radius P^G^, and as it rolls along the evolute into successive positions, P,(7„ P,C„ etc., the motion of the point P is continually normal to its direction. It will also be seen that the radius of curvature of the in- volute at each point is equal to the distance PC from P to the point of contact with the evolute. The conception may be made clearer by conceiving the rolling line to be represented by a string which is wrapped around the evolute. The involute is then formed by the mo- tion of a point on the string. The general method of determining the involutes of given curves involves the integral calculus. PART II. THE INTEGRAL CALCULUS. PART II. THE INTEGRAL CALCULUS. CHAPTER I. THE ELEMENTARY FORMS OF INTEGRATION, 117. Bejinition of Integration, Whenever we have given a function of a variable x, eay M = F{x), we may, by differentiation, obtain another function of x, which we call the derimd function. In the integral calculus we consider the reverse process. "We have given a derived function F'{x), and the problem is: What function or functions, when differ- entiated, will have F'{x) as their derivative? Every such function is called an integral of F'{x), The process of finding the integral is called integration. The operation of integration is indicated by the sign / , called " integral of," written before the product of the given function by the differential of the variable. Thus the ex- pression ' F'{x)dx P -means: that function whose differential with respect to x is F'(x)dx. 202 THE INTEGBAL CALCULUS. Calling M the required function, then if we have we must also have u = fF'{x)dx=f^dx. As examples: Because d{x') = %xdx, we have / ^xdx = x'. Because dijxx' ■\-hx-\-c) = {2ax + i)dx, we have / {2ax -\-b)dx = ax^ -{- bx-{-c. And, in general, if, by differentiation, we have dF{x) = F'{x)dx, we shall have / F'{x)dx = F{x). 118. Arbitrary Constant of Integration. The following principle is a fundamental one of the integral calculus: If F{x) is the integral of any derived function of the va- riable X, then every function of the form F{x) + h, h being any quantity whatever independent of x, will also be an integral. This follows immediately from the fact that h wUl dis- appear in differentiation, so that the two functions F(x) and F{x) + h have the same derivative (cf. § 24). The same principle may be seen from another point of view : Since the problem of differentiation is to find a func- tion which, being differentiated, will give a certain result, and since any quantity independent of the variable which may be added to the original function will have disappeared by differentiation, it follows that we must, to have the most TEM ELEMBNTABT FORMS OV INTEGRATION. 203 general expression for the integral, add this possible but un- known quantity to the integral. The quantity thus added is called an arbitrary constant. But it must be well understood that the word constant merely means independent of the variable with reference to ■ which the integration is performed. It follows from all this that the integral can neyer be com- pletely found from the differential equation alone, but that some other datum is needed to determine the arbitrary con- stant and thus to complete the solution. Such a datum is the value of the integral for some one value of the variable. Let F{x) -\-. h be the integral, and let it be given that when X = a, then the integral = K. "We must have, by this datum, F(a) + h=K, which gives h = K — F{a), and thus determines h. Eemaek. Any symbol may be taken to represent the ar- bitrary constant. The letters c and h are those most gener- ally used. We may affix to it either the positive or the nega- tive sign, and may represent it by any function of arbitrary but constant quantities which we find it convenient to intro- duce. It is often advantageous to write it as a quantity of the same kind as the variable which is integrated. 119. Integration of Entire Functions. Theokem: I. The integral of any power of a variable is the poioer higher by unity, divided by the increased exponent. In symbolic language, we have /j-n + l x'dx = — —r + h, n+1 jpn + l For, by differentiating the expression — — - -\- h, we have ^ w + 1 204 THE INTEGRAL CALCULUa. Theorem II. Any constant factor of the given differen- tial may be written before the sign of integration. In symbolic language. J'aF'{x)dx = aJ'F'{z)dx This is the converse of the Theorem of § 33. By that theorem we have d{aF{x)) = adF{x), from which the above converse theorem at once follows. In the special case « = — 1 we have y*- F'{x)dx = fF\x)d{- x) = - jF'{x)dx. Hence the corollary: If the integral is preceded by the nega- tive sign we may place that sign before either the derived function or the differential., Theoeem III. If the derived function is a sum of several terms, the integral is the sum of the separate integrals of the terms. In symbolic language, y(X+ r+ Z+ . . .)dx = J Xdx-\- J Ydx-\- f Zdx^ . . This, again, is the converse of Theorem II of § 23. The foregoing theorems will enable us to find the integral of any entire function of a variable. To take the function in its most general form, let it be required to find the integral u= I {ax'" + Sa;" -\- ex" -\- . . .)dx. By Theorem III., U= j as^dx -\- I bafdx + / C3^dx -f , o . . THE ELEMENTARY WORMS OF INTEGRATION. 205 By Theorem II., / ax^dx = a I x'^dx; etc. etc. ; and by Theorem I., / jgm + l x'^dx = — — j- + A,; etc. etc. By successive substitution we then have where 7t„ 7^„ h„ etc., are the arbitrary constants added to tha separate integrals. Since the sum of the products of any number of constants by constant factors is itself a constant, we may represent the sum aA, + 57t, + ch^ by the single symbol li. Thus we have jiax'" + 5a;" + ca;" + . . .)dx _ ««:'»+ ' Sa;"+' , ca;^+' EXERCISES. Form the integrals of the following expressions, multiplied by dx: I. x'. 2. x'. 3. a;"'. 4. «"". 5. ax*. 6. Ja;°. 7. ax~\ 8. Sa;~^ 9. fta; + 5. 10. ax' — c. 11. aa;' + ca;. 12. ax' — co^ 13. a:*. 14. a;». 15. x-i. 16. aa;-*. w ah ,1 17. aa:*— Ja;-*. 18. wa;* — — . 19. —,——,. 20. a-\ — ,. 130. The Logarithmic Function, An exceptional case of Theorem I. occurs when w = — 1, because then n -\-l = 0, and the function becomes infinite in form. But since d'los: X = — = x~*dx, ° X (*) 206 THE INTEGRAL CALCULUS. it follows that we haye for this special case J x~^dx = / — = log a; + 7i. (a) Let c be the number of which /* is the logarithm. We then have log a; + A = log a; + log c = log ex. We may equally suppose A = — log c = log -. c Then log a; + A = log -. c Hence we may write either rdx - 1 y - = log ex, /dx , x — = log -; X ° c c being an arbitrary constant. We thus haye the principle: The arbitrary constant added to a logarithm may be introduced by multiplying or dividing by an arbitrary constant the number whose logarithm is ex- pressed. 131. We may derive the integral (a) directly from Theo- rem I., thus: In the general form /j.n + 1 x'dx = — r-5- + h let us determine the constant h by the condition that the in- tegral shall vanish when x has some determinate value a. This gives — -— + /t = 0; .-. h= 1-^. n -\-l n-\-l Thus the integral will become / x^dx = — , TEE ELEMENTAMT FORMS OF INTE&BATION. 207 in which a takes the place of the arbitrary constant. This expression becomes indeterminate for to = — 1. But in this case its limit is found by § 71, Ex. 5, to be log x — log a. Thus we have / 2/ x^dx = log X — log a = log — , as before, log a being now the arbitrary constant, 133. Exponential Functions. Since we have d{d') = log a-a^dx, it follows that we have I log a. a'dx =.0," -J- Ti, or, applying Th. II., § 119, to the first member and then di- viding by log a, d'-\-h /. log a which we may write in the form / a^dx — = f- h, log a ' because = is itself a constant which we may represent by h. 133. The Elementary Forms of Integration. There is no general method for finding the integral of a given differen- tial. What we have to do, when possible, is to reduce the differential to some form in which we can recognize it as tha differential of a known function. For this purpose the fol- lowing elementary forms, derived by diSerentiation, should be well memorized by the student. We first write the prin- cipal known differentials, and to the left give the integral, found by reversing the process. For perspicuity we repeat the forms alrea,dy found, and we omit the constants of in- tegration. 20S08 THE INTEGRAL OALCULUS. .•^(y + >) ={n + l)y''dy, .-d-logy =-^, .• d'siny = cos ydy, . • tZ" cos y = — sin ydy, .•d'tany = aec' ydy. d'cot y _ ^y sm' y' , tan y J .■a-sec« = -dy, " cos y ^ • <Z"sin<~'' 2/ = , • fZ-cos'~"2/ = <Zy 1/1-2/' dy Vi-f' ..^.tan<-).2/=i^-„ . • J-smh<~"2/= a" log ady, dy fray =1^;. (1) /| =log2/. (2) / COS ydy = sin y. (3) / sin ydy = — cos y. (4) tan y. (5) - cot y. (6) r dy ^ J cos' y r dy J sin' sm ^ /tan y<?y cos y r -dy _ = sec y. (7) sin<-"'y. (8) cos<-'>2/. (9) «/ 4/1-2/' .Ja^dy log a' • (11) V/+1 . • .f-r^ = Sin li<-'>2^ = log (y + Vf+1), (12) <f-COSh<-"«= — ;== V2^'- / ■ d-isinM~^^y= dy = COS h<-«2/ = log {y + 4/2/'- 1). (13) 1-2/" ■••/r^,-' =tanh<-V = ^log^^. (14) INTBQItALS BEDUOIBLE TO ELEMENTARY F0SM8. 209 CHAPTER II. INTEGRALS IMMEDIATELY REDUCIBLE TO THE ELEMENTARY FORMS. 134. Integrals Reducible to the Form I y'dy. The fol- lowing are examples of how, by suitable transformations, we may reduce integrals to the form (1). Let it be required to find I (a-\- xYdx, We might develop (a + a;)" by the binomial thorem, and then integrate each term separately by applying Theorem III., § 119. But the following is a simpler way. Since we have dx — d{a + x), we may write the integral thus: Ha + xYd{a + x). It is now in the form (1), y being replaced by a -j- a;. Hence J^a + xYdx=^^±^^-+h. (1) In the same way, f{a - xYdx = - f{a - xYd{a - x) = h - (" ~f^" —. To take another step, let us have to find /"{a + bxYdx. We have dx = j-d(lix) = Td{a -\- hx). , Hence, by applying Th. II., fia+bxYdx=^/(a+hxYd{a+bx)= i^±M.^ + j,^ (3) 210 THE INTEGRAL CALCULUS. We might also introduce a new symbol, y ^a-{-hx, and then we should have to integrate y'^dy with the result in § 123. Substituting for y its value in terms of x, we should then have the result (2).* These transformations apply equally whether n, a and b are entire or fractional, positive or negative. EXERCISES. Find: i. j{a + xydx. 2. y 3(a — xydx. 2,. I {a — 2xydx. 4. / (a + x)-*dx. 5. I (a — x)-^dx. I {a-\-mx)~'dx. "]. I {a — mxfdx. 8. / {a — mo^~^dx. /dx n dx f dx (a + xY' '°V (a - xY' ^^'J I IS 17 18, 19 (« + «)'• J {a-xY' J{a-^xy I {a'\- xydx. 13. / (a + nxydx. 14. / (a + x'yxdx. «/ \ »' ' a; a; V t/ (a — a;)" ^ IC^"^^' + (a - a:)' + (a - a;)V '^'^' t' \(a — mxy {a — mxy {a — mxy) I (a-\-lx-\- cx'){b + 2cx)dx. f{a + I)X + cafyib + 2cx)dx. r {b-\-2cx)dx J (a + bx -\- cx'y * The question whether to introduce a new symbol for a function whose differential 'is to be used must be decided by the student in each case. He is advised, as a rule, to first use the function, because he then gets a clearer view of the nature of the transformation. He can then replace the function by a new symbol whenever the labor of repeatedly writing the function will thereby be saved. INTBGRALa BEBUCIBLE TO ELEMENTABT FORMS. 211 135. Application to the Case of a Falling Body. We have shown (§ 33) that if, at a time t, a body is at a distance z from a point, the velocity of motion of the body is equal to dz the derivative -:-. Now, when a body falls from a height under the influence of a uniform force g of gravity, unmodi- fied by any resistance, the law in question asserts that equal velocities are added in equal times. That is, if z be the height of the body above the surface of the earth, and if we count the time t from the moment at which the body began to fall, the law asserts that dz . . . the negative sign indicating that the force g acts so as to diminish the height z. By integrating this expression, we have z:=zh- igf. (5 ) Here the constant h represents the height z of the body at the moment when ^ = 0, or when the body began to fall. From the definition of h and z, it follows that 7j — 2 is the distance through which the body has fallen. The equation (b) gives h — z = igt". (c) Hence: The distance through which the tody has fallen is jiropiortional to the square of the time. At the end of the time t the velocity of the body, meas- ured downwards, is, by (a), equal to gt. If at this moment the velocity became constant, the body would, in another equal interval t, move through the space gt X t = gf. Hence, by comparing with (c) we reach by another method a result of § 33, namely: hi any period of time a lody falls from a state of rest through half the distance through which tt would move in the same period with its acquired velocity at the end of the period. 212 THE INTEGRAL GALCUZm. 136. Reduction to the Logarithmic Form. Let us have to find /mdx ax-\-o Since dx = —d{ax) = —d(ax + 5)> we may write this expression in the form /m d{ax + S) a ax-\-b ' and the integral becomes m pdiax -\-V\ m , ax + h u= - -^ ~jf = - log —-, aJax-\-o a ° c e being an arbitrary constant. EXERCISES. Integrate the following expressions multiplied by dx: ,1 b m I. X + -. 2. -. 3, -. X X X 1 ^ wi o. x + r ^' 2x- r ex — _m^ „ ffl_ a' '• ^ + '^ • 2ax + b' ^' %bx + a'' 3? -^Jcx a-\-b m — n 10. — ; — ; — i— . II. ; 7. 12. . 4 + « ooj+o mx — n ^. ^ n xdx p xdx Note that xdx = ld{x') = i<Z(l + x'). p xdx ,/•»;' dx /'log a; <?» Note that log x— =^\ogxd. log a;. /»loff(l + w), p xdx n xdx INTEGRALS BEDUCIBLE TO ELBMENTABY F0BM8. 213 I'/iT. Trigonometric Forms. The following are examples of the reduction of certain trigonometric forms: /cos mx dz ■= — I cos mx dUnx) = — sin mx + h. mj ^ ' m /sin mx dx = — I sin mx d(mx) = h cos mx. mJ ^ ' m I cos (a -\- mx)dx = — / cos (a -f- mx)d{a -j- mx) _ sin (a + ?nx) — -j- /I. m /, -, /»siD.xdx nd'cosx tan xdx= I = — / J cos X J cos X = h — log cos X — log c sec x, where h = log c. In the same way, / cot xdx = log c sin a;. /dx n 1 dx pd'tan x , -. = / 7 = / -T — = log c tan X. sm X cos a; J tan x cos a; ^ tan z ° /dx 1 p dx 1.1 -: — = o / -• — i 7- = log c tan -X. sm a; 2 J sm -Ja; cos -Ja; ° 2 / dx _ n dx _ ^ /;r a; \ cos x~ J sin (i;r — a;) ~ ^ \4 2 /" EXERCISES. Integrate : I. (1 + cos y)dy. 2. (1 — e sin u)du. 3. cos 2y dy. Ans. i I cos 2yd{2y) = i sin 2y. 4. sin 2y dy. 5. cos my dy. 6. sin y cos y dy. Ans. i I sin 2yd{2y) = — J cos 2y. 7. tan 2a; dx. 8. cot 2a: dz. 9. 2 cos' X dz. Ans. J 0- + cos 2z)dz = a; + A -f ^ sin 2a;. 214 THE INTEGRAL CALCULUS. lo. 2 sin* xdz. 1 1. tan 3y dy. cos « (iv , rdi'i- + sin «) , /i , • \ 1 + sm y J l-\-smy o v . sin tf <?« sin y dy ■^ 1 + cos y cos y dy , 15. q -V-^. 16. 1 — sm y cos' a; — sin' x . 17. ; — J, oa;. 18. sm %x dx dx 19. . 20. 1 — cos y' sin 3y dy cos 2/ sin 2a; rJr cos' X — sin' X 21. -: — dx cos mx sin ma; sm mx cos ma; 22. sin [mx + o)<?a;. 23. cos (a — nx)dx. 24. tan nx dx. 25. tan (2a; — a)dx. ^ dx dx dx 20. -; -, r. 27. TT r. 26. —. J r. sm (a — x) cos (0 — ma;) sm (a — nx) cos »i«(i!/ sin nydy sec' a;t?a; 20. ;^ -^ . -50. -■^— ^. •51. T . ^ a-{-s\D.ny a — cos ny a— m tan a; 138. Integration of -i— — j awtZ -; ;. (Z ~T* 3/ CI — X We see at once that the first difEerential may he reduced to that of an inverse tangent ; thus, dx 1 dx i4) Hence x' + a' a'x' , ^ ax' ' a' + ^ J' + ^ d- /'_^_ = L r-JL, = L tan<- « i + J a' + ay" a'^ £ , 1 « <^ «' + (1) "We find in the same way /'_,^ = ltanh<-)^ + A = llog«^±^, (2) ,/ a' — x' a a %a '^ a — z ^ c being an arbitrary constant factor. INTEGRALS REDUGIBLB TO ELEMENTABT FORMS. 215 dx 139. Integrals of the form / — + bx -\- ex'' The reduction of integrals of this form depends upon the character of the roots of the quadratic equation cx' + bx+a = 0. (1) I. If these roots are imaginary, the integral is the inverse of a trigonometric tangent. II. If the roots are real and unequal, the integral is the inverse of an hyperbolic tangent. III. If the roots are real and equal, that is, if the above ex- pression is a perfect square, the integral is an algebraic frac- tion. Dividing the denominator of the fraction by the coefficient of x', the given integral may be written dx \f , ^ bx , a c c («) Writing 2p for — and q for — , the expression to be inte- c c grated may be reduced to one of the forms of § 128, thus: dx dx d{x -\-p) x' + 2px"+q ~ (^ +!>)' + ~q-f~{x +pY +'q^'' ^' The three cases now depend on the sign oi q — jo'. I. If q—p' is positive, the roots of (1) are imaginary and the form is the first of the last article with x-\-p in the place of X, and g' — ^' in the place of a'. Hence we have x' 4- %px +~q ~ J {x + pY + q-p' = , ^ ^ tan<-') f +^ +A. (1) Vq -y Vq-p' ^ ' Comparing this result with (a), we see that this integral may be reduced to its primitive form by changing p into 216 THE INTEGRAL CALCULUS. 5- — and q into — . Substituting and reducing, we have dx 1 p dx /dx _ 1 /" a -\- bx ■\- cx^ ~ c J , x' +-X + - c c x + ^ :l._A_tan<-» ^^« c '^ c 4c' '^ c 4c' ; tan< *' — + h. (3) V4ac - /^' ¥4:00 - b' II. If g —p' IB negative, that is, if 4ac — b' is negative in (3), the expression (2) will contain two imaginary quantities. But these two quantities cancel each other, so that the ex- pression is always real. When q —p' is negative, we write (b) in the form The integral is now in the form (2) of § 128, and we have tfa; _ /» d{x + p) /ux _ p a(x x" + 2px + q~ J p' — q — : tanh(-« x+p Vp' — q Vp' — q -* Vp — q yp— i—Kp-Vp) Making the same substitutions in these equations that we made in Case I., we find r dx ^ ^ _ a tanh<-» J^±l. J a + bx + cx' Vb' — 4:ac Vb" - iac 1 , Vb'-^ac + 2cx + h ,,, = h ; log c , ■'—•(4) 4^*'-4ac 4/6''-4ac-(2ca;+5) III. If ^' — g' = 0, the expression to be integrated becomes dx We have already integrated this form and found /dx , 1 (-— = *- INTMaBALS BEDUOIBLE TO ELEMENTARY FOBMS. 217 EXERCISES. Integrate the following expressions: dx. dx dx I. x' -2x- 4' dx " («-«)(»■ dx -/?)• ^' a4-2Ja;-a;'' dx (a — x){x —b) x' + Ax + 2' "' x{x — a)' 130. Inverse Sines and Cosines as Integrals. From what has abeady been shown (§ 123, (8) and (9)), it will be seen that we have the two following integral forms: — = sin <-" a; + A = u; (a) f -^— = ecu (- ') ^ + h.' := 7//; (h) where we have added h and h' as arbitrary constants of in- tegration. Comparing the first members of these equations, we see that each is the negative of the other. The question may therefore be asked why we should not write the second equation in the form u' = - f— r^— = A" - sin <- « X, (c) as well as in the form {h). The answer is that no error would arise in doing so, because the forms {b) and (c) are equivalent. Trom {V) we derive X = cos (m' — A') = cos (h' — u'); (d) and from (c), X = sin (A" — u'). (e) Now, we always have sin (a + 90°) = cos a. Hence {d) and («) become identical by putting h" =:h' + 90°, which we may always do, because the value of h" is quite arbitrary. 218 THE INTEGRAL CALCULUS. 131. The preceding reasoning illustrates the fact that integrals expressed by circular functions may be expressed either in the direct or inverse form. That is, if the relation between the differentials of u and x is expressed in the form - dx du = Vl - x" we may express the relation between u and x themselves either in the form u = sin^~"a; + A or in the form x = sin (m — h). So, also, in the form (1) of § 138 we may express the rela- tion between x and u either as it is there written or in the reverse form, X = a tan a{u — h). dx 133. Integration of ■ We have d-- (1) In the same way /'--==^ = cos<-»- + A or A -sin'-"-. (3) •^ Va' — x' « a ^ ' We also have d-- /• ^^ -= /•-^J= = sinh'-«i + A = log -(a; + 4^^+^). (3) INTEGRALS REDUCIBLE TO ELEMENTARY FORMS. 219 d-- J Vz' -a' J J^ _ a = log -(« + V*' - a'). (4) a Integrate the differentials: dx 2 Vc-x' ndy •^ Va' - nY 4' mdz e — 6 " V^a' - m'z' dx 8. Vic' + x' dy 9- ,. .\ -T-- lO, dy Via dx Vcf -{X- dz ay Via ' - m'z' mdx V7^ + mV dx V^ + m'{x dx -ay V(i" -ay- •4c" «a;" -^dx V'4a' + %' dy II. , 12. Vl + c{x - ay 2xdx 13. — - . 14. Va' - z* Va"" - x"" -. — cos xdx 15. If du = —r======^ then sin a; = a cos (u -{■ , \ (t ~~" SlU iC e'dx dx 16. , 17. — , -. Vl -e"^ ^Vl- (log xy „ — sin Kifo cos xdx 1 8- ::i-r^:7r.--- i9. a' + cos' a;' ^ «' + sin" x (x — a)(5?a; (x 4- a)dx 20. -^ 21. Vl-(x-ay Vl + {x + ay 220 THE INTEGRAL CALCULUS. 133. Integration of Every difEerential of Va-\-bx ± ex' this form can be reduced to one of the three forms of the preceding article by a process similar to that of § 129. The mode of reduction will depend upon the sign of the term ex'. Case I. The term ex' is negative. Putting, as before, 16 a P^%-e' 'i = -e' we have ^a-\-'bx — ex' = Vc Vq-\- 2px — x' = Vc Vp' + q— (x—p)'. Then, comparing with (1) of § 132, we find r dx _ J_ /• d{x—p) '^ Va+'bx - ex' ~ Vc'^ Vf + q- (x-p)' = —— S1T1 < '' , ■^ — = — — R1T1 ' '>^ — (1) V^ ^p' + q Vc V¥+lac In order that this expression may be real, p' -{■ q qt b' -\- iac must be positive. If this quantity is negative the integral will be wholly imaginary, but may be reduced to an inverse hyperbolic sine multiplied by the imaginary unit. Case II. The term ex' is positive. We now have Va + bx+ ex' = Vc V{x + py + q —p'- /dx _ 1 /" ^(.^ + P) Va + bx+ ex' ~ VeJ V(z+py + q-p' = —. log G{x-\-p^ i/x' + 2px + q) Vc = -,- log ^2cx + b + 2c* Va + bx+ ex'). Because Cis an arbitrary quantity, the quotient of by 2c3 is equally an arbitrary quantity, and may be represented by the single symbol O. Thus we have dx 1 /■ Va-\-bx-\- ex' c* log 0{b+2cx+2 Vc Va+bx+cx').{2) INTE0BAL8 REDUCIBLE TO ELEMENTARY FORMS. 221 EXERCISES. Integrate: dy ^ dy Via' + Uy - f V(a + y){b- y) ydy dy V8 - iy' + y*' ' VaY - by + V' cos ddd , cos 6d0 S- -7=======.. 6. V\ - sin 6* — sin" d Vl - sin ^ + cos" sin 6 cos ddd a sin Bdd o. V4 - cos 26/ - cos'' %e Va' - b\l - cos 0)' 134. Exponential Forms. Using the form (11) of § 133, we may reduce and integrate the simplest exponential dif- ferentials as follows: /I /» a"" a'^dx = - / a'^dOmx) = -^, \- h. (1) mj ^ mloga ^ ' fa'+'-dx = fa'+"d{x + b)= ^ + h. (2) /I „ „mx+b a'^ + ''dx=- /a'^ + '>d(mx + b)=—^ \- h. (3) mJ ^ ' ' mloga ^ ' a-'-^dx = - - / a-'^d(- mx) = -^^-^ . (4) mJ ^ I . mlog a ^ ' EXERCISES. Integrate: I. tf°dx. 2. b^dy. 3. a''~^dy. 4. (o + J)e^<fa;. 5. a^-'dy. 6. a-^dx. 7. (a'' + a-'')«Za;. 8. (a'" — a-'")(Za;. 9. (a + e==)<Za;. a'"— a '^)dx. II. r — j — -. 12. 1 + e"'" * 1 + e"^* '3- gx^e- -^- 14. (1 + a'Ydx. 15. (a"« + a-'^)'<Za:. 16. f'e''''xdx. 17. Cef^'xdx. 18. re-<>^'''-'^)xdx. 222 THE INTEORAL CALCULUB. CHAPTER III. INTEGRATION BY RATIONAL TRANSFORMATIONS. 135. We have now to consider certain forms which cannot be reduced so simply and directly as those treated in the last chapter. Before passing to general methods we shall consider some simple cases. I. Integration of ^ s-^rf«. Any form of this kind, when m is entire, may be integrated by developing the numerator by the binomial theorem. We then have {a + a;)"* _ £^ , wa^-'a ; (m\ a''-'x ' and each term can be integrated separately. If w < »i + 2, and entire, one of the terms of the integral will contain log x. II. Integration of . , -. We may reduce this form to the preceding, by introducing a new variable, 2, defined by the equation e = a -\- ix. mi,- • z — a J dz This gives X = — T — ; ax = -r. Substituting these values of z and dx in the expression to be integrated, it 'becomes {z — a)'"dz Jm + l^i. J which may be integrated by the method of the last article. xdx III. Integration of — r-^ — — — 5. We reduce the denomi- INTMOBATION BY RATIONAL TBAN8P0BMATI0N8. 223 nator to the form ± {f — q) ± {x+pY a&ia% 139. Then, putting, for brevity, V =f - q. z = x+p, which gives dx — dz, the integration will have to be performed on an expression of the form b' ± z'~ ~ W~±? ~ F±l"'' Each of these terms may be integrated by methods already given (§§126, 128). The process is exactly the same if we have to find (a + bx)dx /(a -f- oxy pY EXERCISES. Integrate: /I 1\* {x — aydx \a ^1 dx X' X x'dx x'dx •'• (a - xy '" {l+x'Y dx fi {^ + «)'^^ '' xf-+^-r \a xl x^dx ^' (a' - a;')''" xdx zdz 8, (a - xf ' x'dx W x" I 9' a' + (S - xY '"• (a + zf + (a - zY (y — b)dy {z — c)dz ■ iy-by + {y + bY a^-az + z^' (x - a)dx (y + a) dy '3- ^{x - b) ■ '4. «»_(y+-6)5- z'dz z'-dz 224 THM INTEaRAL CALCULUS. 136. Reduction of Rational Fractions in general. A ra- tional fraction is a fraction whose numerator and denominator are entire functions of the variable. The general form is 2'o + ?.« + ?,»" + •••+ ?»«" ~ D ' If the degree m of the numerator exceeds the degree n of the denominator, we may divide the numerator by the de- nominator until we have a remainder of less degree than n. Then, if we put Q for the entire part of the quotient, and R for the remainder, the fraction will be reduced to D ^^ D If we have to integrate this expression, then, since Q is an entire function of z, the differential Qdx can be integrated by § 119, leaving only the proper fraction -jr. Now, such a fraction always admits of being divided into the sum of a series of partial fractions with constant numerators, provided that we can find th^oots of the equation D = 0. The theory of this process belongs to Algebra, but we shall show by ex- amples how to execute it in the three principal cases which may arise. Case I. The roots of the equation D = all real and un- equal. Let these roots be a, ft, y ... 6. Then, as shown in Algebra, we shall have D^{x- a){x - /3){x -y). . . (x-e). We then assume D x-a^ x-^^ x-y^ " ' ' A, B, C, etc., being undetermined coeflBcients. To deter- mine theni we reduce the fractions in the second member to the common denominator D, equate the sum of the numera- tors of the new fractions to R, and then equate the co- efficients of like powers of x. INTEOtBATION BY RATIONAL TBAmFOBMATIONS. 225 As an example, let us take the fraction a; + 3 dx. X — X We readily find, by solving the equation x' — x = 0, x' — X =: x(x — l){x -\- 1). Assume a: + 3 A B C x' — X X X — 1 X -\-l _ {A + B + C) x' + {B - 0) X - A ~ x' — X Equating the coefficients of powers of x, we have ff Qp / A+B+C= 0; B-C = 1; A = - 3; whence B = 2 and C = 1. Hence x + 3 x'-x' _ 3 2 1 1 ~ X ' x-1 1 x + l and then, by §130, J) // ^j>f- y ^, a^-^. •u>c 'r-%±^dx = - 3/^ + 2/^ + T-^ 'J X' —X d X J X — \ tJ X-\-\ = - 3 log a: + 2 log (a; - 1) + log (2; + 1)+ log (7 G{x + V){x-\y x' log- Integrate: {x — Vjdx '• x' -x-Q' xdx X" + X + 1 5" x'+x' -6x' {x- + 2x*)dx ^" x' + 2x''-Sx' x'dx ^' k' — (a + b)x + ab' EXERCISES. xdx 4- 6. 8. (a; + x')dx ix-l){x+l){x-2)ix+2y x'dx {x* ■\-x^)dx x{x - l)(x + l)(x - 2)" dx x^ — (a-\- h^x^ + ahx' 226 TEE INTEGRAL CALCULUS. Case II. Some of the roots equal to each other. Let the factor X — a appear in D to the wth power. Then, if we followed the process of Case I., we should find ourselves with more equations than unknown quantities, because the n fractions X — a X — ax — a would coalesce into one. To avoid this we write the assumed series of fractions in the form (a; - «)»^ (a; - «)»-' ^ • • • + a; - a + a: -yS + ^^^^ and then we proceed to reduce to a common denominator as before. The coefficients A, B, etc., are now equal in num- ber to the terms of the equation /) = 0, so that we shall have exactly conditions enough to determine them. As an example, let it be required to integrate a;' - 5 , X — X — X -\-l We have a;' - a;' - a; + 1 = (a: - 1)' (a; + 1). We then assume x'-5 A . -5 , C (a; - 1)" (a; + 1) (a; - 1)' ' a; - 1 ' a;+ 1 _ (B + C)x' + (.4 - 20)x -i-A-B+O (a;-ir(a; + l) We find, by equating and solving, A = -2; B^+2; 0=-l. Hence a;' -5 -^ _j_ ^ 1 (a; - l)"(a; + 1) (« - 1)' ' a; - 1 x-\-l' INTEaBATIOHr BT RATIONAL TRANSFOBMATIONB. 327 The required integral is -^A-')-*+^/j#i-/jf-i = ^4l^ + 2 log (x - 1) - log(a; + 1) + log C 2 , , Cix-iy 'log- — X —1 ° X -\-l EXERCISES. Integrate: dx dx x{x+iy ■ x\x-iy x'dx dx ^' {x - \)\x + %Y ^' {x - ay{x - by (a + x)dx (a — x)dx ^' x\x - af x\x + ay{;x - h)' Case III. Bnaginary roots. Were the preceding methods applied without change to the case when the equation i) = has imaginary roots, we should have a result in an imaginary form, though actually the integral is real. We therefore modify the process as follows: It is shown in Algebra that imaginary roots enter an equa- tion in pairs, so that if x ^ a -\- (Si (where i = V — 1) is a root, then x = a — fti will be another root. To these roots correspond the product {x- a- ^i){x - a + ^i) = (a; - a)' + /?». By thus combining the imaginary factors the function D will be divided into factors all of which are real, but some of which, in the case of imaginary roots, will be of the second degree. The assumed fraction corresponding to a pair of imaginary roots we place in the form A-[- Bx {x-ay+^' 228 TBE INTEORAJu CALCULUS. and then proceed to determine A and B as before by equa- tions of condition. We then divide the numerator A -\- Bx into the two parts A + Ba and B{x — a), the sum of which is ^ + Bx. Thus we have to integrate /• A + Ba _, p B( x - a)dx , . J (x- ay + z?'**^ + y (a - of + /J'- ^"^ The first term of (a) is, by methods ah-eady developed, A -\- Ba , , ,.x — a _^_tan<->-^, and the second is iB log {{x- ay + ^'). We therefore have, for the complete integral, / A ■\- Bx A-\-Ba, , ,.x—a — tan'-'>- BXERCISES. I. / — ; :T-dx. 2. / -i -. .»/ x — 1 "/a; — 1 The real factors of the denominator in Ex. 1 are (a' + l)(x + l)(x — 1). We resolve the given fraction in the form A + Bx C D a' + l ^"a! + l'*'a!-l' all and find it equal to , . , + ^7^ H 5 • Then the integral is found to be i log («!» + 1) + log (a!» — 1). The factors of the denominator in Ex. 3 are «— 1 and 3?-\-x-\-\ = r dx p (a:' + \)dx ^' J x'-\-l' ■ ^' J x' -'ix + 4" Note that a; + 3 is a factor of the denominator in (4). INTEGRATION BY RATIONAL TRANSFORMATIONS. 229 13 7. Integration ly Parts. Let u and v be any two functions of x. We haye d(uv) dv , du az dx dx By transposing and integrating we have fufjlz = uv-fv^£dx + h, (1) which is a general formula of the widest application, and should be thoroughly memorized by the student. It shows us that whenever the differential function to be integrated can be divided into two factors, one of which f;^-^^;] can be integrated by itself, the problem can be reduced to the inte- gration of some new expression \v-^dx\. The formula may be written and memorized in the simpler form / udv = uv — I vdu, (2) it being understood that the expressions dv and du mean dif- ferentials with respect to the independent variable, whatever that may be. It does not follow that the new expression will be any easier to integrate than the original one; and when it is not, the method of integrating by parts will not lead us to the integral. The cases in which it is applicable can only be found by trial. The general rule embodied in the formulae (1) and (2) is this : Express the given differential as the product of one function into the differential of a second function. Then its integral will he the product of these two functions, minus the integral of the second function into the differential of the first. 230 TBE INTEGRAL CALCULUS. EXAMPLES AND EXERCISES IN INTECRATION BY PARTS. 1. To integrate x cos xdx. We have cos xdx = d- &ia x. Therefore in (3) we have u = x; v = emx; and the formula hecomes / X cos xdx = I xd'smx = a; sin a; — / sin xdx = a; sin a; + cos a; + h, which is the required expression, as we may readily prove by differentiation. Show in the same way that — 2. I x sin xdx = — a; cos a; + sin a; + A. 3. I X sec' xdx = X tan x — (what ?). 4. I X sin X cos xdx = — \x cos 2a; + i sin 2a; + h. 5. /log xdx = a; log a; — / xd-logx= a; log a; — a; + 7i. 6. The process in question may be applied any number of times in succession; For example, / x' cos xdx = I a;'^-sina; = x' Bin x — 2 I x sin xdx. Then, by integrating the last term by parts, which we have already done, fx' cos xdx = x' sin a; + 2a; cos a; — 3 sin a; + h. 7. In the same way, fx' cos xdx = / x'd- sin a; = x' sin x— 3 / x' sin xdx; fx' sin xdx = — / x'd- cos a; = — a;' cos a; -j- 3 / a; cos xdx. INTEGRATION BT RATIONAL TRANSFORMATIONS. 231 Then, by substitution, / x' cos xdx = {x' — 6a;) sin x + (3a;' — 6) cos x-\-'h. 8, In general, I x" cos xdx = /a;"rf-sina;=a;"sina; — n /a;"-' sin xdx; — / a;""' sin xdx = I a;"~'<Z-cos a; = a;"-' cos X — {n — 1) I x"-" cos a;da;; — Jx"-' cos a;rfa; =— a; "-'sin a; +(» — 2) / a;"-' sin a;<?a;; /a;"-' sin a;rfa; =— a;""' cos a; +(w — 3) / a;"""* coaxdx. etc. etc. etc. Then, by successive substitution, we find, for the required integral, {a;"— w(w— l)a;"-«+M(«-l)(>i— a)(M— 3)a;"-*— . . .} sin a; + j«a;"-^ — m(w — 1) (w — 2)a;"~' + • • ■ } cos a;. 9. In the same way, show that / x" sin xdx = {—a;"4-«(»*—l)^"~*— >»(»*— l)(w—2)(M—3)a:"~*-f. ..( cos a; + jwa;"-' — n{n — l)ln — 2)a;"~' +. • •} sin x. /I Z' a;** "''^ a;" log xdx = — -— - / log xd' (a;" + *) = log x fl -p It/ w — p X 1 /'a;"** a;» + i , a;"+' 7 / dx = — — :; log X — 7 — , ■ ,, . n + lj X n + 1 ^ (» + 1) II. rxe-'^dx=- r^xd-{e-'") = - ^^+i- A-^rfa;. e~'^dx = . Jlence jxe~^dx = — xe' 232 THE INTEGRAL CALCULUS. 12. To mtegrate Q^6~'"dx when m is a positive integer, we proceed in the same way, and repeat the process until we re- duce the exponent of x to unity. Thus, x'^e-'^dx = — :-- h - / x'^-^e-'^dx. a aj Treating this last integral in the same way, and repeating the process, the integral becomes a;'»e~'" mx'^-'-e-'^ m(m — l)x'^-''e-'" a =^ ^ etc. a a a -ax ^-^i(a"2:"+OTa"— ■a;"'-'+»w(OT-l)a'»-''a;»-'+ . . .+ml). a 13. From the result of Ex. 5 show that y"(log xydx = x{r -21 + 2) +h, where we put, for brevity, I = log x. 14. Show that, in general, if we put w, =y (log a;)" dx, then M„ = a;Z" — WM„_i; and therefore, by successive substitution, M„ = xil' — nl"-^ + n{n — 1)1"-' - . . . ± nl) + h. 15. Deduce {m + 1) fpx^dx = Fx'" + ^ - J'x'^ + ^dP. 16. Show that if P Pdx = Q, then / Px^'dx = ga;" — n I Qx'^-^dx. Also, if we have / Qdx - R; I Rdx = S, etc., then fPa^dx = Q^ — nRx''-'^ + n{n — l)^"-^ — etc. INTEGRATION OF IRRATIONAL DIFFERENTIALS. 233 CHAPTER IV. INTEGRATION OF IRRATIONAL ALGEBRAIC DIFFERENTIALS. 138. When Fractional Powers of the Independent Vari- able enter into the Expression. In this case we may render tie expression rational by reducing the exponents to their least common denominator, and equating the variable to a new variable raised to the power represented by this denominator. Example. If we have to integrate 1 + ^^. — ■ — -.dx, 1 + a;* then, the L, 0. D. of the denominators of the exponents being 6, we substitute for x the new variable z determined by the equation x = z', which gives dx = 6z^dz. The differential expression now reduces to ,z' + z\ z' + l By division this reduces to dz. 6{Z' -Z' + Z' + Z'-Z- l)dz + ^j-p-^ + -5-p^. Integrating and replacing z by its equivalent, .r*, we find n+^dx = u - u + u + u - u - ex^ i/l_[_a;* 7 5 4 '3 2 + 3 log (.T* + 1) + 6 tan<-« x^ + h. 234 THE INTEGRAL OALOULUa. If the fractional exponent belongs to a function of x of the first degree, that is, of the form ax + I, we apply the same method by substituting the new variable for the proper root of this function. Example. To integrate {a-\-hx)^dx 1 + (a + bx)' We put {a + bx)^ = z; a + bx = z'; , 2zdz ax = —J—. The expression to be integrated now becomes 2z'dz _ 2( _ dz \ b{l + z')~b[ z'+lJ' of which the integral is ~{z- tan(-'>2 + A) = J- j {a + bx)*-twi^-»{a + bx)*+h \ . EXERCISES. Integrate: x^dx x^dx 1 — a;* I. r— I — . 2. !• 3- \dx. {a - x)'dx Jfl-xfdx_ g l + «-^,to. '*■ 1 + a - «• ^- 1 _ (« _ xf ' (a - xy 7. i^^dx. 8. ^^^.. 9. J?£Z1_«M xo. l±i^^dz. „.'illi£±f)-V 1 + (2 - c)* 1 + (a; + a)' y a; , a;' - 12. — aa;. 13. ^dx. X* 14. ]_ Vx ~ z* (x - a)* - (a: - g)* ^^ (a; - a)* + (a; - a)* INTEGRATION OF IRRATIONAL DIFFERENTIALS. 235 139. Gases when the Given Differential contains an Irrational Quantity of the Form Va-\-bx-{- ex'. It is a fundamental theorem of the Integral Calculus that if we put B = any quadratic function of x, then every ex- pression of the form F{x, VR)dx, {F(x, VH) being a rational function of x and i/R), admits of integration in terms of algebraic, logarithmic, trigonometric or circular functions. But if R contains terms of the third or any higher order in x, then the integral can, in general, be expressed only in terms of certain higher transcendent func- tions know as elliptic and Abelian functions. We have three cases of a quadratic function of x. First Case : c positive. If c is positive, we may render the expression rational by substituting for x the variable z, de- termined by the equation Va -\- bx -\- ex' = Vc{x -\- z); .' . a ■{• ix -{- ex' = ex" -\- 2cxz -\- cz'. mi,- ■ cz' — a This gives X = j^Tg- ; («) a — bz + cz'j ,,. Va + ix + cx' ^ - Va^^^. (.) By substituting the values given by (a), (5) and (c) for the radical, x, and dx, the expression to be integrated will become rational. Second Case ; a positive and c negative. If the term in x' is negative while a is positive, we put Va-^bz — ex' = Va-\- xz. We thus derive a; =: — ;— | ; (a) 236 THE JNTEQBAL CALCULUS. 2( Vaz' -Vac- hz), ,,, dx = -5^ ^,^^, dz; (b) The substitution of these expressions will render the equa- tion to be integrated rational. Third Case : a and c both negative. If the extreme terms of the trinomial are both negative, we find the roots of the quadratic equation — a -\- bx ~- ex' = 0, which roots we call a and /?. We then have — a-\-bx — ex' = e{a — x) (x — /3), and we introduce the new variable z by the condition V— a-\-bx — ex' = Ve{a — x) (x — /3) — Ve{x — a)z, which gives z = ^. ' ^ ; (a) »^- {z' + iy ' ^"^ ^ ^a + bx-ex' ^ ^£^>; (e) z +1 ^ ' substitutions which will render the equation rational, 140. We have already integrated one expression of the dx form just considered, namely, —^==^^= without ration- Va -\-bx-\- ex' alization. There is yet another expression which admits of being integrated by a very simple transformation, namely, de^- —^'—^ . r Var' -\-br — 1 This is the polar equation of the orbit of a planet around the sun. To integrate it directly, we put INTEOBATION OF IRRATIONAL DIFFERENTIA '^^^, 1 , dz z = —; dr = -.. We thus reduce the expression to — dx f: Va-\-hz — x" Proceeding as in §133, Case I., we find the value of the integral to be /: dr , ,. %x — h , ,, 2 — br = cos*" ' — — = cos^~ ' ^^^ r Var' + br-l V4:a + b'' r Via + b' Thus, d-7r = cos(-» -^^JL=, r VU + V n being an arbitrary constant. Hence = cos {a — Tt). r Via + V Solving with respect to r, we have, for the polar equation of the required curve, 3 . . ^ ~ 5 + 4/(4a ■+ V) cos ((9 - n)' ^"^ This can be readily shown to represent an ellipse. The polar equation of the ellipse is, when the major axis is taken as the base-line and the focus as the pole, *" ~ 1 4- e cos ^ ~ 2 , 2e ' ~R 5\ + "7i T\ COS a(l — 6 ) a(l — e ) Comparing with (a), we have o a(l — e°) = 7- = parameter of ellipse =^; or e = -^-^ — 7 = eccentricity of ellipse. •^38 THE INTEGRAL CALCULUS. Irrational Binomial Forms. 141. General Theory, An irrational binomial differen- tial is one in the form (a + ix'^yz'^dx, (1) in which m and n are integers positive or negative, while p is fractional. To find how and when such a form may be reduced to a rational one, let the fraction p, reduced to its lowest terms, be r — ; and let us put y=(a + bx'^y. (2) This will give, when raised to the rth power and multi- plied by x^dx, {a + bx^yx'^dx = x-y'tiz, (3) We readily find, from (2), ix" = y' — a; (a) dx = hnx' x^'fdx = 7-a;'"-"+*y'" + »~Wy; or, substituting for x its value from (a), a^y^dx = ^(^-^Y^^y^ + '-'dy. (4) This last differential will be rational if is an in^ n teger, which wiU be the case if ^ is an integer. We shall call this Case I. To find another case when the integral may be rationalized, let us transform the given differential (1) by dividing the bi- nomial by a;" and multiplying the factor outside of it by x"", which will leave its value unchanged. It will then be mTEQBATION OF IRRATIONAL DIFFERENTIALS. 239 {i-\-ax- ""Yx " + ''"dx, (1') ■vrhich is another differential of the same form in which n is changed into — n and m into m + np. Hence, by Case I., this form can be made rational whenever — is an n integer; that is, when — — — \-pia such. We have, therefore, two cases of integrability, namely: Case I. : when — ^i^ = an integer. n Case II. : when — — — |-^ = an integer. Eemaek. It will be seen that all differentials of the form r (a + Ix^Yx'^dx must belong to one of these classes, because — '^^— is an integer when m is odd, and — ^^ H o ^^ sach when m is even. In this statement we assume r to be odd, because if it is even the original expression is rational. 143. If, in Case I., the integer is + 1, that is, if wi + 1 = n, then the expression can be integrated immediately. For (4) then becomes the integral of which, after replacing y by its value in (3), be- comes (« + hxrx^-^dx = ^j^f^^ + c. (5) Again, if the integer in Case II. is — 1, we have m -\- 1 -\- np = — n, or JM + wp = — M — 1. The expression (1) reduced to the form (1') will then be (5 + aa: -")''« -»-'<?a; = - (5 + aa;-")" — d{l) -\- ax-"). 240 TEE INTEGRAL CALOULUS. which is immediately integrablej and gives by simple reduc- tions 143. Forms of Reduction of Irrational Binomials. Al- though the integrable forms can be integrated by the substi- tution (2), it will, in most cases, be more convenient to ap- ply a system of transformations by which the integrals can be reduced to one of the forms just considered. The objects of these transformations are: I. To replace mhy m -\- n or vi — n; II. To replace ;? by ;? + 1 or ^ — 1. 144. Firstly, to replace m by wi + n. Let us write, for brevity, X= a + Ja;", which will give dX = Inx " ~ ^dx, and the given differential will be X'x'^dx, which, again, is equal to ^.m — n + l /j.ln — n + 1 X''dX= ~' , , .■ <^(X^+')- hn bn{p-\- 1) Integrating by parts, we have Since X^ + ' = X^{a + W) = aX" + iXPx", the last integral in the above equation is the same as a f X^x '"-''dx + b C X'x'^dx, of which the second integral is the same as the original one. Making this substitution in (a), and then solving the equa- INTEOBAriON OF IRRATIONAL DIFFERENTIALS. 241 tion so as to obtain the Talue of / X^x^dx, we find / X'x'^dx = T7 ; — ^— r^ — TT •^^^^ / ^'^a;''^«'a;. {A) 'J o(np-\- m -\-l) o{np-\-m-\-l)t/ Thus the given integral is made to depend upon another in which the exponent of x is changed from m to m — n. By reversing the equation we make the given integral depend on one in which the exponent is increased by n. To do this we change m into m -{• n all through the equation (^4), thus getting I j,Px'«+''dx=^, j 1 i^^-TT — \ , , -,^ / ^'a;'"^?^;- «/ o{np-^m-}-n+l) o(np-{-m-\-n->rl)<y Solving with respect to the last integral, we find fx'-x-dx = 4^; - Hm+E±-+}) fx^x'^^^dx. (B) The repeated application of [A) and {B) enables us to make the value of the given integral depend upon other in- tegrals of the same form, in which »i is replaced by m-{-n; m-\-'^n; etc.; or by m — n; m — %n; etc. 145. Next, to obtain forms in which p is increased or diminished by unity, we express the given differential in the form X'-x'^dx = X'd m + l \m -)- 1/ Integrating by parts and substituting for dX its value })nx''~^ dx, we have fx^x-^dx = ^f^' _ JUlS- fx^-H-^^-dx. IV) Now, we have „m + » _ „n.„» _ a:"(X- «) _ Xa;" aa;"* 242 THE INTECHIAL GALGULUB. and therefore, by multiplying by X^-^dz, Substituting this value in (S), and solying as before with respect to / X'afdx, we shall find J wp + m + 1 Mp + m + 1^/ ' X ' in which p is diminished by unity. If we write ^ 4-1 forjo in this equation, the last integral will become the given one. Doing this, and then solving with respect to the last integral, we find fx^x-dx = - ?--p^, + "^+;+"+^ rX^^x-dx.(D) J an{p-\-l) an{p-\-l) J ^ ' By the repeated application of the formula (C) or (2>) we change p mkop — 1, ^ — 3, ^ — 3, etc., or p into p -\-l, p -{■%, p -\-Z, etc. 146. To see the efEect of these transformations, let us put, in the criteria of Cases I. and II., § 141: I. — ■ — = I, an integer. n 11. — ■ \-p = i', an integer. Then when we apply formula {A) or (5), since we replace m by »j — M or wi -}- re, we have, for the new integers: n ^ mT^ + 1 ., ^^ n ^ It is also clear that by (C) and {D) we change II. by unity. Thus, every time we apply formulae {A), (B), (C) or (D) we change one or both of these integers by unity, so that we may bring them to the values unity treated in § 142. INTEGRATION OF IRRATIONAL DIFFERENTIALS. 243 147. Case of Failure in this Bedudion. If, in an integral of Case II., i' is positiye, we cannot change it from zero to — 1 by the formula {A) or (C), hecanse, when — — 1- ^ = 0, we have m -\- 1 -\- np — 0, and the denominators in {A) and (C) then vanish. In this case we have to apply the substitution of § 141, without try- ing to reduce the integral farther. EXAMPLES AND EXERCISES. 1. To integrate (a' ± x'fdz. We see that if we diminish the exponent -J by unity, we shall reduce the integral to a known elementary form of § 132. So we apply (C), putting m = 0; n = 2; p = i', a = a^; S = ± 1. Then (C) becomes J(,a±x)dx- g +2y(a.±^y We therefore have, from § 132, y (a' + x'fdx = \ I x{a' + x'f + «' log ^{x+{a^+ x'f) [ ; /*(«' - x')^dx = ~ j x{a' - x'Y + a' sin^-" |- + A I . Deduce the following equations: 2. / (c' — x') xdx =h — i{c' — a;')'. 3.f{o' + xrxdx =h+(^+^fl\ 4. /{C + x'Yx-^dx = h + (^y^'. _dx ^ ^^ _ ( c' + x')* x'ic' + a;")* ~ ' " o^x 244 THE INTBQBAL CALCULUa. 7. y (a' - x')^dz = ia;(a' - x'f + K Bin <-« -. Here apply formula ((7); in the following (4). 8. fil - x^fx'dx ^ h - (J + A)(i _ ^.)». 9. To reduce and integrate (1 + x')^x'dx. Here m = S; n = 2; p = i; m -\- 1 = i = 2n. "We can therefore reduce the form to Case I. by a transformation of m into m — n, for which we may use either (a) or (A) of § 144. Using (a), we have y (1 + a')* a^dx = (^+^')"°' ' _ ly (1 ^ j,j)8 a!;^. The last integral can he immediately found, and gives for the required integral 4(1 + »')«(!!» -A(l +»')'■ (o) Using {A), we should find y(l + «»)* !,^dx = (1 + »')« g - ^g), (J) a form to which (a) can be immediately reduced. The student will remark that the form (a) is reduced to (A) because in the former the exponent of X is increased by 1, which often makes the integration inconvenient. But when this increase of p does not in- terfere with the integration, we may use (as) more easily than {A). 10. To reduce and integrate (1 + x^)*x^dx. Applying {A), we find y (1 + x^f afdx = <1 + ^'^ _ *y (1 + a,5)* aJijte. A second application repeats the form (J) above, thus giving y(i+.')V<*.==(i+.^)^g-S+j«^). 11. Reduce and integrate (1 + x')*x'^dx, where m is any positive odd integer, and show that INTEOBATION OP IBBATIONAL FUNOTIONB. 345 y (1 + a;')^ dx ■ j/a;"'-' (m-l)a:"'-' , (m-l)(OT-3)a;'^ \ ^ ^ ' \m+2 (m + %)m ' {m+2)m{m-2) Bemabk. Where the student is ■writing a series of transformations he will find it convenient to put single symbols for the integral expressions which repeat themselves. Thus: rx''afdx = {l); /'x''a!'"-"(&=(a); etc. Thus the equations of reduction in the present example may be written ^^'- m + 3 -^^+2^''^^' XVrJ_»^3 m m etc. etc. 12. Deduce the result 5ffl° 138 ■j a;(a' + a;')* + a' log C(a: + V^T+x^) f . 246 TEE INTEGRAL CALCULUS. CHAPTER V. INTEGRATION OF TRANSCENDENT FUNCTIONS. When the given differential contains trigonometric or other transcendent functions of the variable more complex than the simple forms treated in Chapter II., no general method of redaction can be applied. Each case must therefore be studied for itself. 148. To find the integrals / e"" cos nxdx and / e™" sin nxdx. (1) Since we have the integration by parts of these two expressions gives /e"" cos wa; , M /> ^^ . , e"" cos nxdx = \-- e"^ sin nxdx; /e"™ sin wa; n f ,^ , e"" sm nxdx = / e"" cos nxdx. m mJ Solving these equations with respect to the two integrals which they contain, we find /e'^im cos nx + n sin nx) e""* cos nxdx — — ^^ ■ -' m' + n' /„, , , e""(»» sm nx — n eos nx) e"" sin nxdx = — ^ 5-1 — ~ — ^J (3) which are the required values, Eemaek. These integrals can also be obtained by substi- tuting for the sine and cosine their expressions in termg <jf imaginary exponentials, namely, INTEGBATION OF TBANBCENDENT FUNCTIONS. 247 2 cos «« = 6"** + e ~"^, a sin nx = T-(e"^ — e""^), and then integrating according to the method of § 134. The student should thus deduce the form (2) as an exercise. 149. Integration of sin" x cqs" xdx. This form is readily reducible to that of a binomial, and that in two ways. Since we have cos xdx — d'sia. x, cos a; = (1 — sin' a;)*, we see that the integral may be written in the form / n- 1 (1 — sin' a;) a wa!^ xd'saxx; or, putting y = sin x, f{l-y')^y'-dy. (3) By putting « = cos a; we should have, in the same way, / m — 1 (1 - z') ~»~z''dz, (4) which is still of the same form, and is always integrable by the methods already developed in Chapter IV. If either m or w is a positive odd integer, then by develop- ing the binomial in (3) or (4) by the binomial theorem we shall reduce the expression to a series containing only posi- tive or negative powers of x, which is easily integrable. We can also, in any case, transform the integral so as to in- crease or diminish either of the exponents m and n by steps of two units at a time, as follows: sin"" X cos" xdx = cos " ~ ' a; sin" xd' sin x „ , ^in^+'a: = cos"~'a!a , 1^. w-f 1 Then, integrating by parts, we have 248 TEE IN2EQBAL CALCULUS. A —T /sin"" + ^ a; cos""' a;rfa;. (5) m -\-lJ ^ ' I sin" X cos" xdx _ cos"~^ X sin "■ + ':!; Because sin "" + ^2; = sin" x{l — cos" x), the last term is equivalent to 'I — I/*-™ -«• w — !/>.„ „, — r-^- / sm" 2; COS""* a; -^ I sm" a; cos" xdx. m + It/ J» + It/ The last of these factors is the original integral. Trans- posing the term containing it, we find (ot + n) I sin" a; cos" xdx — sin" + 'a; cos"~*a; + (w — 1) / sin" a; cos ""' xdx, (6) in which the exponent of cos x is diminished by 3. We may in a similar way place the given difEerential in the form • ™ 1 7 cos " + ^ a; — sm""^ xd -— — , « + 1 and then, proceeding as before, we shall find (m + n) I sin" a; cos" xdx = — sin"~'a; cos" + ' x -\- {m — 1) I siw^-' X cos" xdx, (7) thus diminishing the exponent of sin x by 3. By reversing these two equations we get forms in which the exponents are increased by 2. "Writing n-\-2 f or m in the first, and ?k + 2 for m in the second, we find {n + 1) / sin" X cos" xdx = — sin" + ' a; cos" + ^ x + {m + n + 2)/' sin" a; cos " + ^ xdx; (8) (m + 1) / sin" X cos" xdx = sin"*^ a; cos" + ^ x -\- {m+n + 2) J sin "' + ^ x cos" xdx, (9) INTEGBATION OF TRANSCENDENT FUNCTIONS. 249 150. Special cases of I sin" x cos" xdx. If m is zero and n is positive, we derive, from (6), /„ , sin a; COS""' a; , n — 1 r> „ , , cos" xdx — / cos"~* xdx; n ' 11 J /' » a J sin a; cos"-" » , w — 3 /» „ , -, \^^) /cos"-^xdx = -s ^ / cos"-* xdx; etc. etc. etc. The integral to be found will thus become that of cos xdx when n is odd, and that of dx, or x itself, when n is even. The given integral is then found by successive substitution. We find in the same way, frctm (7), /. „ , cos a; sin "-^ a; , m — 1 /• . „ „ , sm™ xdx = / sin™ "xdx; m m J /■ ™_5 7 cosa;sin"^a; , ?w — 3 /> . „^ , sm ""-"»& = -^ s / sin"^a;rfa;; etc. etc. etc. From (8) and (9) we derive similar forms applicable to the case of negative exponents. (11) EXERCISES. I. / sin' X cos' xdx. Ans. \ cos' X — ^ cos' x 2. 1 sin' X cos' xdx. Ans. \ sin' x — \ sin'a;. Poos' xdx •^V sin* X ' . 3 sin' x — 1 Ans. „ ■ . . 3 sm' X 4. y sin' X tan' xdx. /"cos' X -, 5. / V — ^—dx. ^ J tan' X 6. / e'" sin ^ydy. 7. re'' + '' cos (x + h)dx. 8. / e'" sin y cos ydy. g.J'e-^coa' {y -\- a) dy. JO. Derive the formulae of reduction tan"*"' Aan™ xd9^ = ' — xi~ "~ AaiJ""^^ ^^^\ II 12 250 TBE INTEGBAL CALCULUS. and hence / tan" xdz = r / tan"~^ xdx. These equatious may be obtained independently by putting tan" x = tan " - 2 a!(sec' x—1); or they may be derived from (5). Hence derive the integrals: / tan' xdx = ^ tan' x — log c sec x. (Cf. § 127) . / tan* xdx = ^ tan' a; — tan x -\- x -\- h, 13. For all odd positive integral values of n, /•, „ , tan'-'a; tan"-'^ , , , / tan" xdx = s- + ... ± log c sec x. J M — 1 n — 3 14. When 71 is positive, integral and an even number, /», „ , tan"-'a; tan"~'a; , , , / tan" xdx = = — h • • • ± tan x ± x. J n — \ M — 3 15. When the exponent is integral, odd and negative, /', „ , cot"~'a; , cot"-'a; , , / iwx'^'xdx = z ^ . . . ± logcsma;. J n — 1 n — 3 16. When the exponent is integral, even and negative, />, „ , cot"-^a; , cot"-'a; , , ^ / tan-" xdx = — = ... ± cot a; q: a;. J n — 1 n — 3 /■ » 7 cos xf . , , 4 . , , 4-3\ sm xdx = ---I sin a; + o sm ^ + oT^ )• i8. I Bin' xdx coBxf . , , 5 . , , 5-3 . \ , 5-32; = ^[sinx + ^smx + -^^smx)+^:^^. 19. / sin" X cos" xdx = -- / sin" 2xdx cos 2z sin"~' 2a;,w — l/>._„„, — s^m h -5s — / sm"- " 2xdx. 3"+' n 3"w J INTEOBATION OP TRANSCENDENT FUNCTIONS. 251 151. To integrate , . . ; — = ^— = du. '' m sm X + n cos x Dividing both terms of the fraction by cos" x, noticing that = d'tan X and writing t = tan x, we find cos X dt " = / — 573— i — 5- (12) J mt -{-n ^ The integral is known to be (§ 138) J^tan<-«^^, mil n so that we have u = f , . , "^r , — = — tan'-" - tan z + h, (13) J VI sm X -\- n cos x mn n > ' \ / or tan x — — tan mn(u — h). m ^ ' 153. Inteq ration of — --r=^ . rt + cos «/ We reduce this form to the preceding one by the following trigonometric substitution: a = «(cos" ly -j- sin" |?/); h cos «/ = S(cos" ^y — sin" |y); by which the expression reduces to the form J {a - I) sin" iz/ + (a + h) cos" i^y' ^ ' which is that just integrated, when we put a; = ly; m = Va — i; n = Va-\-i. We therefore have f^^ = ^_tan<-V^tan^ + A. (15) J a-\-bQosy ^a' — b' a+ i '^ ' ^ ^ 252 THE INTEGRAL CALCULUS. 153. If, in the form of §151, w" and «' have opposite signs, or if in § 152 we have b > a, imaginary quantities will enter into the integrals, although the latter are real. If, in the first form, the denominator is m' sin' x — n' cos" x, we shall have, instead of (12), the integral J m't' - n' ~ ¥nj mf^n ~ %nj W+n ^§ ^^^^ mt -\- n ■M^^-:+''- f 2mn ° mi — n Hence, corresponding to (13), we have the result dx _ 1 m tan ^ + n . m' sin" x — n' cos' x ~ 2mn ° m tan x — n ^ ' If, now, in § 152, 5 > «, we write (14) in the form _ 3 /• ^iy_ J {b — a) sin' i«/ — (a + b) cos' ^y' and instead of (15) we have the result / dy ^/f I ^ log ^'^ ^ *^^^ ^■^+ '^'^ + 1 (17) a+b cos y "*" V^,'_fl' ^ Vi-atan ^y- Vb -\- a 154. Infrgration of sin mx cos nxdx. Every form of this kind is readily integrated by substitut- ing for the products of sines and cosines their expressions in sines and cosines of the sums and differences of the angles. We have, by Trigonometry, sin mx cos nx = ^ sin {m -{- n)x -\- ^ sin (in — n)x. Hence , cos (w/ + n)x cos (m — n)x , , Sm mx cos nxdx = -r- ; r ;t7^ T T ^ 2(m -\- n) 2{m — n) We find in the same way sin (m. 4- 7i)x , sin (m — n)x , , cos mx cos nxdx = —rr^ ; r tt? — - — r — h " 2(to + n) 2(m — n) sin (m -4- n)x , sin (tii — n)x , , sm mx sm nxax = ^ — , — r- H — ^7^ 7- + A, 2{m 4- V) 2{m — n) f f INTEGBATION OF TBAN80ENDBNT FUNCTIONS. 253 155. Integration ly Development in Series. When the given derived function can be developed in a convergent series, we may find its integral by integrating each term of the series. Of course the integral will then be in the form of a series. The development of many known functions may thus be obtained. EXAMPLES AND EXERCISES. I. We may find / sin xdx as follows: We know that •/ X' , X° X' a;" a;' x' sin aiflla; = A + ^ - ^ + g| - etc.. which we recognize as the development of — cos x with an arbitrary constant h -\-\ added to it. Of course we may find / cos xdx in the same way. dx 2. To integrate 1 + a;" (1 + a;)-' = 1 - a; + a;' -a:' + ■/ 1+a; Now, we know that Jytt — ^°S (1 + *)• Hence (a) is the development of log (1 -f x), when we put % = log 1 = 0. The series (a) is divergent when a; > 1. In this case we may form the development by the binomial theorem in de- scending powers of x, thus: ix + l)-^ = x-^ — x-' + x-^-x-^+ . . . . Hence we derive, when a; > 1, log(. + l) = log. + l-^4 + ^l^_l^+.... 254 THE INTEaBAL CALCULUS. The arbitrary constant is zero because, when x is infinite, log (a; + 1) — log X is infinitesimal. %. To find / ' = sin <~" x in a series. Hence r dx . , ,, , 1 a;V 1-3 a;', 1-3-5 a;' , The arbitrary constant is zero by the condition sin<~" = 0. This series could be used for computing w by putting x = i, because i = sin 30° = sin -. But its convergence would be much slower than that of some other series which give the value of Tt. dx rive the expansion 4. From the equation / — = log {x-^- Vl + x") de- t/ Vl-\-x' 1 x" , 1-3 x' 1-3-5 a;' log ^^+vr+^)=x--.^ + ^-^.^ - ^.^.^ + . . 5. By expanding , = £Z-tan<~" a;, derive tan<-» x = x-ix' + ix'-^x''+ . ... Derive: /• '^^ -z,r I a^° 1-3 a:' 1-3-5 x" • J l^r+^'~ 2-5 +2-4-9 2-4-6 13+* 7. fe-'''dx = h + x-^+^,-^^,+ .... DEFINITE INTEGRALS. 255 CHAPTER VI. OF DEFINITE INTEGRALS. 156. In the Differential Calculus the increment of a variable has heen defined as the difference between two values of that variable. Let us then suppose m to represent any variable quantity whatever, and let us suppose u to pass through the series of values "oJ ^1) ^J> M„ . . • Un. Then we shall have Jw„ = w, - w„; ^M, = Mj — M^; ^«, = Ms - M.; Taking the sum of all these equations, we have Au„ + Zu^ + All, + . . . + ^«<„_i = «„ - w,; That is, the difference between the two extreme values of a variable is equal to the sum of all the successive increments by which it passes from one of these values to the other. The same proposition may be shown graphically by sup- posing the variable to represent the distance from the left- hand end of a line to any point upon the line. The differ- ence between the lengths Au, and Au^ is evidently Au^ -\-Au,-\-...+ Au,. , I Am, I AMi I Am, I Au, I An, | .... _j «„ Ui u, «s ttj «5 Since the proposition is true how small soever the incre- ments, it remains true when they are infinitesimal. 256 TEE INTEGRAL CALCULUa. 157. Differential of an Area. Let P^PP' be any curve whatsTer, and let us inyestigate the differential of the area swept over by the ordinate y XP. Let us suppose the foot of the ordinate to start from the position X„, and move to the position X. During this motion XP sweeps over the area X^P^PX, the magnitude of which will depend upon the distance OX, and will ^'°- ^''• therefore be a function of x, which represents this distance. Let us put u = the area swept over; y = the ordinate XP. Then, if we assign to x the increment XX', the corre- sponding increment of the area will be XPF'X'. Let us call y' the new ordinate X'P'. It is evident that we may always take the increment XX = Ax ao small that the area XPP'X' shall be greater than yAx and less than y' Ax or vice versa. That is, if y' > y, as in the figure, we shall have yAx < Au < y'Ax. Now, when Ax approaches the limit zero, y' will approach y as its limit, so that the two estremes of this inequality yAx and y'Az will approach equality. Hence, at the limit, du = ydx. (1) That is, the area u is such a function of x that its differen- tial is ydx, and its derivative toith respect to x is y. From this it follows by integration that : / ydx + h (2) is a general expression for the value of the area from any initial ordinate, as X^P^ to the terminal ordinate XP. DEFINITE INTBQBAL8. 257 158. The Conception of a Definite Integral. Suppose the area X„P„PX=m to be divided up into elementary areas, as in the figure. This area will then be made up of the sum of the areas of all the elementary rect- angles, plus that of the o^ triangles at the top of s'"'- «• the several rectangles. That is, using the notation of § 156, we have « = y,Ax, + y^Ax^ + y,Ax^ + • • • + y„_,^a;„_, + T, T being the sum of the areas of the triangles; or, using the notation of sums, « = n — 1 u= 2 y^Axi + T. io Now, let each of the increments Axt become infinitesimal. Then each of the small triangles which make up T will be- come an infinitesimal of the second order, and their sum T will become an infinitesimal of the first order. We may therefore write, for the area u, x=OX x=OX u = lim. 2 yAx = 2 ydz. x=OXo x=OXo That is, u is the limit of the sum of all the infinitesimal products ydx, as the foot of the ordinate XP moves from X„ to X by infinitesimal steps each equal to dx. Such a sum of an infinite number of infinitesimal products is called a definite integral. The extreme values of the independent variable x, namely, 6>X„ Ea;„ and 0X= a;,, are called the limits of integration. The infinitesimal increments ydx, whose sum makes up the definite integral, are called its elements. 17 258 TEE INTEGBAL CALCULITS. 159. Fundamental Theokem. The definite integral of a continuous function is equal to the difference between the values of the indefinite integral corresponding to the limits of integration. To show this let us write (f>{z) for y, and let us put, for the indefinite integral, '*(f)(x)dz = F(z) + c. /' Now, as already shown, this is a general expression for the area swept over by the ordinate y = (p(x), when counted from any arbitrary point determined by the constant c. If we count the area from X„P„, the area will be zero when x = x„; that is, we must have F{x,) +c = 0, which gives c = — F{x„). If we call a;, the value of x at X, we shall have u = Area X,P„PX = F{x,) + c = F{x,) - F{xX (3) which was to be proved. We therefore have a double conception of a definite in- tegral, namely: (1) As a sum of infinitesimal products; (2) As the difference between two values of an indefinite integral; and it will be noticed that the identity of these two concep- tions rests on the theorem just enunciated. Notation. The definite integral is expressed in the same form as the indefinite integral, except that the limits of inte- gration are inserted after the sign / above and below the line; thus, r(f>{x)dx means the integral of cp{x)dx taken between the limits x„ and x^, the first being the initial and the second the terminal limit. DEFimTE INTEaSALS. 259 Example of the Identity of the Two Conceptions of a Defi- nite Integral. The double conception of a definite integral just reached is of fundamental importance, and may be further illustrated analytically. To take the simplest possible case, consider the definite integral Jx~ adx, a being a constant. By definition this means the sum of all the products adx -\- adx -\- adx + . . . , as X increases from «„ to x^. The sum of all the doi^a must be equal to a;, — x^ (§ 156), Hence a{dx -\- dx -{■ dx -\- dx -\- . . . ) = a{x^ — a;„). But we have for the indefinite integral / adx ■■ and the definite integral is therefore, by the theorem, ax^ — ax, or a{x^ — a;„), as before. 160. Differentiation of a Definite Integral with respect to its Limits. — Because the definite integral / ydx = u means the sum of all the products ydx as x increases by infinitesimal increments from the lower limit x, to the upper limit a;,, or u = y^dx + y'dx + y"dx + . . . + y^'^^dx, therefore, assigning an increment dx^ to the terminal limit x^ will add the infinitesimal increment y^dx^ to u (see Pig. 48). That is, we shall have du = y,dx„ or ^ = y, = 0(a;j. (4) In the same way, increasing the initial limit x, by dx, will take away from the sum the infinitesimal product y,dx,, so 260 THB INTEGRAL CALCULUS. that we shall have |=-,.= -0K). (5) The equations (4) and (5) give us the derivatives of the definite integral u= I <p{x) . dx with respect to its limits x^ and a;„. 161. Examples and Exercises in finding Definite Inte- grals. The fundamental theorem gives the following rule for form- ing definite integrals: 1. Form the indefinite integral. 2. Substitute for the variable with respect to which we inte- grate, firstly, the upper limit of integration; secondly, the lower limit. 3. Subtract the second result from the first. The difference will be the required definite integral. 1. / x'dx = ix^' — ix,\ 2. j xdx = 1(5' — a"). 3. / xdx = i. 4. / sin xdx = — cos n -\- cos = 2. 5. / cos xdx = sin ^Tt. 6. / azdz = ia{a* — S'). ' sin 2xdx. 8. / cos 2xdx. t/45° sin' xdx. 10. / cos' xdx, «/o II. / X Bin xdx. 12. / z COB zdz. t/o t/o 13. I z' sin zdz. 14. / 2' cos «<?«. DEFINITE INTEGBALS. 261 15. / z' cos 2zdz. 16. /* z' sin %zdz. t/O t/o /^dx „ />" , — . 18. / nz^dz. /^ dx p^' dz —, ,• 20. / -5 -. 21. / cosa;<?a;, 22. / siuxdx. 25. / (a; — «)«». 26. / ydy. t/a — b t/a — X ^1 — a; _a + c 27. / (is — l)'(?a;. 28. / [x — a){x — c)dz 31. r^'smaxdx. 32. f^ cos {a + x)dx. COS (x + ?/)</a; = sin 2y. V 34. Show that r f{x)dx = — C f{x)dx. 35. Deduce / c~''dy = l. /■" 1 36. Deduce / e~'"'c?w = -. t/o a 37. Deduce / e^dy = 1. t/ — QO 38. Deduce / e- ^''ydy = ^. 39. Deduce f -— — ^ = tt. t/_oo J- + 2 «^ ^2 ;r 40. Deduce / 41, Deduce / - = n. ^ J- a Va' - z' •+" dz 262 THE INTEGRAL CALCULUS. 163. Failure of the Metliod when the Function becomes Infinite. It is to be noted that the equivalence of the two conceptions of a definite integral does not necessarily hold true unless the function ;/ or (p{x) is continuous and finite between the limits of integration. As an example of the failure of this condition, consider the function y {x - af the curve representing which is shown in the margin. The indefinite integral is u - 1 ydx = c — X a MA Fig. 49. To find the value of this integral between two such limits as and k, h being any quantity OM less than a, we put a; — and x = h, and take the difference as usual. Thus 1 1^ k a u ■■ (5) a — k a a{a — k)' !Now, if we suppose k to approach a as its limit, so that a —k shall become infinitesimal, then the area tc will increase without limit, as we readily see from the figure as well as by the formula. But suppose h > a; for example, k = 2a. Then the theorem would give ,2a 1 _1 __3 /o ' a a a a negative finite quantity; whereas, in reality, the area is an infinite quantity. The theorem fails because, when x ^= a, y becomes infinite, so that ydx is not then necessarily an infinitesimal, as is pre- supposed in the demonstration. DEFINITE INTEGRALS. 263 163. Change of Variable in Definite Integrals. When, in order to integrate an expression, we introduce a new vari- able, we must assign to the limits of integration the values of the new variable which correspond to the limiting values of the old one. Some examples will make this clear. Ex. 1. Let the definite integral be dx £ /o a-\-x Proceeding in the usual way, we find the indefinite integral to be log (a + x), whence we conclude £^^=^^%^'^-^^%'^^^^^'^- But suppose that we transformed the integral by putting y'B.a-{-x; dy — dx. Since, at the lower limit, a; = 0, we must then have y — a for this limit, and when, at the upper limit, a; = a, ve have y — 2a. Hence the transformed integral is rdy 'y' which we find to have the same value, log 2. Ex. 2. u — I ^ sin x{l — cos x)dx. We may write the indefinite integral in the form / sin xdx -\- I cos xd(cos x). In the first term x is still the independent variable. But, as the second is written, cos x is the independent variable. Now, for a; = 0, cos x = 1; and for x = -^, cos x = 0. Hence, writing y for cos x, the value of u is u= 1^ sin xdx -\- j ydy = 1 ~ ^ = ^. 264 THE INTEGRAL GALGULU8. Bekabe. The variable with respect to which the integra- tion is performed always disappears from the definite integral, which is a function of the limits of integration, and of any quantities which may enter into the difEerential expression. Hence we may change the symbol of the variable at pleasure without changing the integral. Thus whatever be the form of the function <p, or the original meaning of the symbols x and y, we shall always have jT <p{x)dz =J^ 4>{y)dy =£ "^{y + a)dy, etc. 164. Subdivision of a Definite Integral. The following definitions come into use here: 1. An even function of a; is a function whose value remains unchanged when x changes its sign. 2. An odd function of x is one which retains the same absolute value with the opposite sign when x changes its sign. As examples: cos x is an even, sin x an odd, function. Any function of x* is even; the product of any even func- tion into X is odd. It is evident, from the nature and formation of a definite integral, that if we have a sum of such integrals, r (p(x)dx 4- /* <t>{x)dx + / (f>{x)dx +... + / <l){x)dx, J a «/6 Jo Jg in which the upper limit of each integral is the lower limit of that next following, this sum is equal to <t){x)dx. f This theorem may often be applied to simplify the expres- sion of the integral in cases where the values of 0(a;) repeat themselves. Theoeem I. If cj){x) is an even function of x, then, what' ever he a, <p{x)dx = 2 / <p{x)dx. DEFINITE INTEGRALS. 265 Because 0(— a;) = ^(a;), it follows that for every negative yalue of x between — a and the element of 0(a;)c?a; will be the same as for the corresponding positive value of x. Hence the infinitesimal sums which make up the value of / cf){x)dx will be equal to those which make up / ^{x)dx. Therefore /a fji nd nf^ ^{x)dx = / 4>{x)dx + / <p{x)dx = 3 / 4>{^)dx, Theorem II. If(l>{x) is an odd function of x, then, what- ever be a, / + » (p(x)dx = 0. a For in this case each element (p{— x)dx will be the negative of the element (j){x)dx, and thus the positive and negative elements will cancel each other. EXERCISES. Show that / e'^'x^dx = I (log — j dz. Substitute X = log -. 2. Show that whatever be the function 0, we have j (j}(sm. z)dz = / <p{cos xdx). As an example of this theorem. '*"« + 5 cos" x^^ _ pi^a -\- I sin" x b sin" X n^^a -t- COS" a; , _ pt^a + Jo a — b COS" X ~Ja a — dx. The truth of this theorem may be seen by showing that to each ele- ment of the one integral corresponds an equal element of the other. Draw two quadrants; draw a sine in one and an equal cosine in the other. Any function of the sine is equal to the corresponding function of the cosine. We may fill one quadrant up with sines and the other with cosines equal to those sines, and then the two integrals will be made up of equal elements. 266 THE INTEOBAL CALCULUS. To express this proof analytically, we replace s by a new variable y — \ic — X, which gives sin x = cos y; dx = — dy; and then we invert the limits of the transformed integral, and change y into x in accordance with the remark of the last article. ^ IT 3. Show that I /(sin x)dx = 2 i^fism x)dx. 4. Show that / 0(sin x) cos xdx — 0. 5. Show that if be an odd function, then / 0(cos x)dx = 0. 6. Show that the product of two like functions, odd or even, is an even function, and that the product of an even and an odd function is an odd function. 7. Show that when is an odd function, 0(0) = 0. 165. Definite Integrals through Integration ly Parts, — In the formula for integration by parts, namely, / udv = uv — vdu, let us apply the rule for finding the definite integral. To ex- press the result, let us put (uv)^ and (««)„, the values of uv for the upper and lower limits of integration, respectivelyj I udv and / vdu, the values of the two indefinite in- tegrals for the upper limit, a;,; fudv and / vdu, the values of the integrals for the lower limit, x„. We then have, by the rule of § 161, Jf udv = I udv — I udv = (m«), — / vdu — {uv), + / ^du = (uv\ — {uv), — I vdu. t/x« DEFINITE INTEQBALS. 267 In order to assimilate the form of this expression to that of a definite integral, it is common to write {uv)\ = (uv), - {uv\. EXAMPLES AND EXERCISES. 1. We have found the indefinite integral / log xdx = X log X — I dx. If we take this integral between the limits a; = and x = l, the term x log x will vanish at both limits, so that {x log a;), - {x log a;)„ = 0. Hence j log xdx — — j dx=— l-\-0=—\. 2. To find the definite integral, I sin" xdx. In the equation (11), § 150, the first term of the second member vanishes at both the limits x — and x = 7t. Hence / sai!^xdx=. / sin^^^aj^^a;. Jo m Jo Writing »i — 2 f or m, and repeating the process, we have m — Z r 5bi'"~^xdx = X i sva.'^~^xdx', Jo m — 2 Jo I sm!^~^xdx — 7 / sin™~'a;(?a;: Jo m — 4:Jo etc. etc. If m is even, we shall at length reach the form I dx = 7t — = Tt. Then, hj successive substitution^ we shall hav§ 268 THE INTEGRAL CALCULUS. Ja m(m — 2)(»i — 4) ... 3 If m is odd, the last integral mil be j sin xdz = + 2, and we shall hare r "sin™ xdx = s ("^ - ^)(^» - 3) ... 2 i/o m(m — 3)(m — 4) ... 3 3. From the equation (6) of § 149 we have, by forming the definite integral and dividing hj m-\-n, /»" /sin"'+'a;cos"~*a;\'' / sin" z cos" xdx = I ; Ja \ m-\-n Iq A "^ — / sin" X COS""' xdx. m + nJo Since sin ?r = sin = 0, the first term of the second member vanishes between the limits, and we have /*"" n — 1 p"" I sin" a; cos" a!<?a; = ; — / sin" a; cos"~*a;£?a;. Jo m + nJo Writing n — 2, and then m — 4, etc., in place of n, this formula becomes f sin" a; cos""' a;<fa; = — ; ^/ sin"a; cos""* a;t?a;: Jo m + n — 2 Jo I sin" X cos""* xdx = — ; r / sin" x cos""' xdx; Jo m + n — 4: Jo etc. etc. If n is odd, the successive applications of this substitution will at length lead us to the form XT 1 sin" X cos xdx = — r^ (sin"+' tt — sin" + ' 0) — 0; and thus, by successive substitution, we shall find all the in- tegrals to be zero, DEFINITE INTEGRALS. 269 If n is even, we shall be led to the form I sin" xdx, which we have just integrated. Then, by successive substi- tution, we find /" sm" X cos" as {n - 1)(to - 3) . ■ ■ 1 {m + n){m + w — 2) . . , (wi + 2), dx 'i)Jo sin" xdx. 4. To find /7-5— i — iw- Jo («' + «) We transform the differential thus: /dx _ 1 />(a;' + a" — a;') <?a; 1^+7^" ~ d'-J («' + d'Y Integrating the last term by parts, we have r x'dx _^ r _Jixdx__ _l r d- jx'+d') J (a;' + ay ~ %J ^(a» + a;")» ~ 2 «/ * (a;" + a")" =\Sxd. (x'+ff")"-^ 2(w-l)"(a;" + «'')"-' 1 — w 1 r dx_ abstituting this value of the Substituting this value of the last term in (a), we have dx _ \ X (a;' +^)» ~ %a\n - 1) (a;" + «')"-» ^ a'V 2(w - V)]J {x^ + «")»-!• Passing now to the limits, we see that the first term of the second member vanishes both for a; = and for a; = oo . We also have 1 2«-3 1- 2(w - 1) 2(« - !)• 270 THE INTB&RAL OALGULTTS. Hence we have the formula of reduction n-^__dx__ _ 2w — 3 /»" dx Jo {x'-\-a'Y~%{n-l)a'Jo (a' + a;"p^i" ^''> We can thus diminish the exponent by successive steps until it reaches 2. The formula {b) will then give dx n X (a;" + ay ~ 2^'Jq ^ + x'~ 4a'" Then, by successive substitution in the form (b), we shall have P'° dx ^ {2n - 3)(2w - 5) ... 1 tt Jo {x' + a')''~ (2n-2)(2n-i). . .2-2a"'-'- ^"^ If in (c) we suppose a = 1, and write the second member in reverse order, we have Jo (1 dx _ 1-3-5 ■ . . (2w-3) jr + x'y~ 2-4:-6 . . . (aw-a)"2" Let us apply to the indefinite integral the formula (A), § 144. We have in this case a = l; b = —1; n = 2; p = — i. The formula then becomes r x^dx _ _ a;"-' Vl — a;' m- 1 f x'^-Hx In the same way P x'^-^dx _ _ x^-Wl-x" TO-3 P x'^-^dx J if\—x^ ~ 'f"— 2 TO — 3t/ 4/i":ir^' Continuing the process, we shall reduce the exponent of x to 1 if m is odd, or to if m is even. Then we shall have Taking the several integrals between the limits and 1, we •^-=i"^ (0) DEFINITE INTEGBALS. 271 note that in (a) the first term of the second member vanishes at both limits, while {b) gives />! xdx _ />! dx _ 1 Jo Vl-x'~ ' J Vl-x'~ ^^' We thus have, by successive substitution, _ p x'''+^dx _ 2n{2n - 8) (2w - 4) 2 y^ + '=Jo |/izr^ - (a«+l)(3w-l)(2w-3) ... 3' a^t^g: _ (2w-l)(2re-3)(2M-5) . ■ . 1 5 yrir^» ~ 2n{2n — 2) (3m — 4) 2 '2 Let us now consider the limit toward which the ratio of two values of y^ approaches as m increases to infinity. We find, from (a), .Vm _ m — l Vm-, m ' a ratio of which unity is the limit. 2fext we find, by taking the quotient of the equations (c), n _ {2-4- 6 . ■ . {2n — 2)-2nY y^ 2 - ts^tt. . (2w - \)\\2n + 1)-^;;:^^' Since, when n becomes infinite, the ratio y^^ '■ .Van + 1 ap- proaches unity as its limit, we conclude that \Tt may be ex- pressed in the form of an infinite product, thus: Tt 4 4' 6' 8' 10" , . ^ ., 2 ^ 3 "3^5 -S^r 7^9 -901 ' ' ' <'^ '^^fi^'*^^- This is a celebrated expression for n, known as Wallis's formula. It cannot practically be used for computing n, owing to the great number of factors which would have to be included. 272 THE INTEGRAL GALCULUB. CHAPTER VII. SUCCESSIVE INTEGRATION. 166. Differentiation under the Sign of Integration. Let us have an indefinite integral of the form u = I (j){a, x)dx — F(a, x), (1) a being any quantity whatever independent of x. It is evi- dent that u will in general be a function of a. We have now to find the differential of u with respect to a. The differentiation of (1) gives d'u _ d<t)(^a, x) dadx da _ d''u ^du ^du , , ., Because -^—7- = ^a^— = -^» j~» ^^ ame, when we consider -3— as a function of x (cf. § 51), Jdu\d^_a^^ dcl>{a,x) ^^^ \da I dxda da Then, by integrating with respect to x, ^ ^ p d<P{a,x) da J da in which the second member is the same as (1), except that 0(a, x) is replaced by its derivative with respect to a. Hence we have the theorem: The derivative of an integral with respect to any quantity which enters into it is expressed hy differentiating with re- spect to that quantity under the sign of integration. SVCCESaiVE INTEasATION. 273 167. This theorem being proved for an indefinite inte- gral, we have to inquire whether it can be applied to a definite integral. If we take the integral (1) between the limits a;, and a;,, and put m„ and m, for the corresponding values of u, we have, for the definite integral, r ^4>(oL, x)dx = F{a, a;,) - F(a, x,) = u,-u, = m,'. Then, by differentiation, <Zm/ _ dF{a, a;,) dF(a, a;„) da da da Comparing (1) and (3), we have (3) rd^{a^ ^^ ^ dF{a, a;) . v da da ' da da whence, if a;, and a;„ are not functions of a, ^^ d<p{a, x) ^^ ^ dF{a, a;, ) _ dFja, x,) ^ Jx„ da da da ' ^ ' Hence from (3) we have the general theorem Da / (t>{a, x)dx = I Da.4>{a, x)dx. That is, the symbols of differentiation and integration with respect to two independent quantities may be interchanged in a definite integral, provided that tJie limits of integration are not functions of the quantity with respect to which we differ- If the limits a;, and a;„ are functions of a, we have, for the total derivative of m/ with respect to a (§ 41), t?M„' _ !du^\ du^ dx^ du„ ' dx^ da ~ \ da ) dx^ da dx„ da' By §160 we have 18 274 THE INTEQBAL CALCULUB. Thus from (3) and (4) we have «?M.' /^idip(a,x), , ,, .dx, ,, ,dx. ... This formula is subject to the same restriction as the theorem for the value of a definite integral; that is, (l>{a, x) and its derivative with respect to a must be finite and con- tinuous for all values of x letween the limits of integration. If this condition is not fulfilled, (5) may fail. EXERCISES. Differentiate: 1. / — ; — with respect to a. Ans. — / 7 — ; — r-.. 2. j{x + aydx with respect to a. Ans. n l{x-\-a)''-^dx. 3. Ma;' -l-a;y)'(?a; with respect toy. Ans. 2 r{x^-\-z''y)dx. 4. r x'dx with respect to a. Ans. a'. 5. / x'dx with respect to a. Ans. 8a'. ajVa; with respect to or. Ans. =a"(2a" + ^—l). And show that we have the same results in the first three cases whether we integrate the differential with respect to a or y, or differentiate the integral. 168. The preceding method enables us to find many integrals, indefinite and definite, by differentiating known integrals with respect to constants which enter into them. Thus, by differentiating with respect to a the integral fef^dx = -6-^+0, 8U00ESSI7M INTEGRATION. 275 we find, after adding the constants of integration, /.'.«^..= g-^-f + !>- + .; etc. etc. wMcli leads to tlie same results as integration by parts, and is shorter. 169. The following is an instructive application of this and other principles. We shall hereafter show that From this it is required to find the value of / e~" ** dy. If we put x = ay, whence dy = — , the corresponding indefinite integral will be f'-"''y=\f' e *' dx. N"ow, when y = ± oo , we have also a; = ± oo . Hence r^\-'''y'dy=ir^\--dx=^. tJ—CXi ^t/— CO d By differentiating with respect to a, and simple reductions, we find and from this, etc. etc. 2x cos ax: 276 TSE mTEORAL CALCULUS. KXERCISES. 1. By differentiating the integrals y cos axdz = - sin ax, fsia axdx = cos ax, twice with respect to a, prove the formulae /• , , fx' 2\ . , 2x I X cos axdx =1 5 1 sm ax -\ — 5 r ■, ■ , (2 x'\ , 2x . I X sm axdx =[—, cos ax + -5 sm ax. J \a al a Thence show that we have Jy^ cos ydy = (y' - 2) sin y + 2y cos y; Jy' sin ydy = {2 - y') cos y + 2y sin y. 2. Prove the formulae: /o 1 />" 1 e'-^dx =-; (b) / x^'dx = - -,; 00 a fj — 00 a K'e-^Ja; = -,; (rf) / x'^^dx = (- 1)"-^. 3. Show that the preceding formulae are true only when a is positive, and find the following corresponding forms when a has the negative sign: /»°° , 1 />" ,1 / e-'^dx = -; I xe-'^dx = -,; i/o a i/o a r x'e-'^dx = -,; C x'e-'^dx = ~r ; etc. Jo a Jo a' 4. By differentiating the form of § 132, namely, dx /(a' {a' - xy with respect to a, show that dx sm (-«: /(«' {a' - xy ~ a'(a' - x')*' SUCCESSIVE INTEORATION. 277 170. Double Integrals. The preceding results may be summed up and proved thus: Let us have an integral of the form u = J<p{x, y)dx, (1) and let us consider the integral J'udy or fy/'Pi^' y)d^j^y> which, for brevity, is written without brackets, thus: J J(p{x, y)dxdy. This expression is called a double integral. Theoeem. The value of an indefinite double integral re- mains unchanged when we change the order of the integra- tions, provided that we assign suitable values to the arbitrary constants of integration. Let us put V = J<p(x, y)dy, u retaining the value (1). The theorem asserts that / udy = I vdx. Call these two quantities U and V, respectively. We then have, by differentiation, dU d^U du ^, , — =v ^^=— = ct>(x dx ' dydx ~ dy ~ ^^ ' "/• Therefore, because of the interchangeability of differentiations, ^dU ^dV 'dx _ 'dx dy ~ dy ' Then, by integration with respect to y, — - — 4-c- dx ~ dx ' 278 THE INTEGRAL CALCULUS. and, by integration with respect to x, U— V+cx + c'. Putting c = and c' — 0, we have U= F, as was to be proved. mi. By the process of successive integration thus indi- cated we obtain the value of a function of two variables when its second derivative is given. The problem is, having an equation of the form ^"^ <l>{x,y), (3) dxdy where 0(a;, y) is supposed to be given, to find u, as a func- tion of x and y. This we do by integrating first with respect to one of the variables, say x, which will give us the value of T— , because the first member of (2) is D~r^. Then we in- dy ' dy tegrate with respect to y, and thus get «. As an example, let us take the equation d'u „ ^ du , - xy , or d.-^ = xy dx. dxdy '^ ' 'dy Integrating with respect to x, we have %=ry+'' (3) h being a quantity independent of x, which we have common- ly called an arbitrary constant. But, in accordance with a principle already laid down (§118), this so-called constant may be any quantity independent of x, and therefore any function we please to take of y. Next, integi'ating (3) with respect to y, and putting Y= = Jhdy, we find u = Ix^y' + F-f X, in which X is any quantity independent of y, and so may be an arbitrary function of x. Moreover, since h is an entirely arbitrary function of y, so is Y itself. SUGOESSrVE INTE&BATION. 279 The student should now prove this equation by difEerenti- ating with respect to x and y in succession. 1'73. Triple and Multiple Integrals. The principles just developed may be extended to the case of integrals involving three or more independent variables. The expression cl){x, y, z)dxdydz fff^ means the result obtained by integrating ^{x, y, z) with re- spect to X, then that result with respect to y, and finally that result with respect to z. The final result is called a triple integral. If we call F{x, y, z) the final integral to be obtained, we have. d^F{x, y, z) ., , and the problem is to find F{x, y, z) from this equation when 0(a;, y, z) is given. Now, I say that to any integral obtained from this equation we may add, as arbitrary constants, three quantities: the one an arbitrary function of y and z; the second an arbitrary function of z and x; the third an arbitrary function of x and y. For, let us represent any three such functions by the symbols [y, 2]> b, ^], [x, «/], and let us find the third derivative of F{x, y, z) + {y, z] + [«, x] + \x, y'] = u with respect to x, y and z. Differentiating with respect to X, y and z in succession, we obtain du _^ dF(x, y, z) d[z, x] d[x, yl _ dx ~~ dx dx dx ' O'u _ d'Fjx, y, z) d'lx,y] ^ dxdy ~ dxdy dxdy ' d'u _ d'F{x, y, z) _ dxdydz dxdydz ' an equation from which the three arbitrary functions have entirely disappeared. 280 THE INTEOBAL CALCULUS. It is to be remarked that one or both of the variables may disappear from any of these arbitrary functions without chang- ing their character. The arbitrary function of y and z, being any quantity whatever that does not contain x, may or may not contain y or z, and so with the others. As an examplCj let it be required to find u = I I I {x — a){y — b){z — c)dxdydz. Integrating with respect to z, and omitting the arbitrary function, we have J fW - "){y - *)(« - cfdxdy. Then integrating with respect to y, ^=J-^{x-a){y-bY{z-oy; which gives, by integrating with respect to x, and adding the arbitrary functions, ?« = i(x - afiy - l)\z - cY + [y, z] + [z, x] + [x, y]. The same principle may be extended to integrals with re- spect to any number of variables, or to multiple integrals. The method may also be applied to the determination of a function of a single variable when the derivative of the func- tion of any order is given. EXERCISBS. 3. / / I xy'z'dxdydz. 4. I j j-^dxdydz. 5. I I I {x — ay{y — b){z — cydxdydz. 6. ff{x~aydx\ 1. J J J(z-\-'hydz\ Ans. (6). ^^{x — ay -\- Ox -\- C, Cand C" being arbitrary constants. SUCCESSIVE INTEGRATION. 281 173. Definite Dotiile Integrals. Let U be any function of X and y. By integration with respect to a*, supposing y constant, we may form a definite integral / Udx= U'. From what has been shown in § 163, Eem., U' will be a function of y, x^ and x^. We may therefore form a second definite integral by integrating U'dy between two limits «/„ and y^. Thus we find an expression f U'dy =11 Udxdy, which is a definite double integral. The limits a;„ and x^ of the first integration may be con- stants, or they may be functions of y. If they are constants, the two integrations will be inter- changeable, as shown for indefinite double integrals. If they are functions of y they are not interchangeable, un- less we make suitable changes in the limits. 174. Definite Triple and Multiple Integrals. A definite integral of any order may be formed on the plan just described. For example, in the definite triple integral r / / (j)(x, y, z)dxdydz the limits a;„ and a;, of the first integration may be functions of y and z; while i/„ and y, maybe functions of z. But z^ and ^j will be constants. So, in any multiple integral, the limits of the first integra- tion may be constants, or they may be functions of any or all the other variables. And each succeeding pair of limits may be functions of the variable which still remain, but cannot be functions of tho§e with respect to which we have already integrated, 282 THE LNTEGBAL CALCULUS. EXAMPLES AND EXERCISES. I. Find the values of / / xy'dxdy and / / xy'dxdy. It will be seen that in the first form the limits of x are constants, and in the second, functions of y. First integrating with respect to x, we have for the indefi- nite integral / xy'dx — ^x'y'', and for the two definite integrals / xy^dx = ^a'y^, / xy^dx = \y\ f/y Then, integrating these two functions with respect to y, we have ££\y-'dxdy = if'y^dy = ^b\ Let us now see the effect of reversing the order of the in- tegrations. First integrating with respect to y, we have ^xy'dy = ixb' = U. Then integrating with respect to x, we have f Udx ^J^Tx^dydx = \a^h\ the same result as when we integrated in the reverse order between the same constant limits. 2. Deduce f ^ f cos (a; + y)dxdy = — 3. SUCCESSIVE INTEGRATION. 283 3. Deduce / / cos {x — y)dxdy = + 4. 4. Deduce / / {x — a){y — b)dxdy = ^a'b'. 5. Deduce f''J"'{x - a)(y - b)dxdy = \{2ab-a'){2ah-h'). 6. Deduce / " / ^ {x—a){y—b)dxdy = a'b — iab'— fa'. 1*75. Product of Two Definite Integrals. Theoebm. The product of the ttoo definite integrals I ^Xdx atid I ' Ydy is equal to the double integral I "' I 'XYdxdy, provided that neither integral contains the variable of the other. For, by hypothesis, the integral / Xdx = C7 does not con- tain y. Therefore U f Tdy^f UYdy = / / XYdxdy, Jva vVo vy„ ux„ as was to be proved. 176. The Definite Integral I e~'' dx. This integral, which we have already mentioned, is a fundamental one in the method of least squares, and may be obtained by the ap- plication of the preceding theorem. Let us put k= r'^'°e-''''dx = % r^'^e-'^'dx^'H r'^"'e-^^dy.{%lU) Then, by the theorem, lc^= 4 r^\-^'dx r^\-''dy =4 r^" Z'+V^'+^'Wy. i/o Jo t/o t/o Let us now substitute for y a new variable t, determined by the condition y = tx. 284 THE INTEGRAL CALCULUS. Since, in integrating with resiject to y, we suppose x con- stant, we must now put dy = xdt. Also, since t is infinite when y is infinite, and zero when y is zero, the limits of integration for t are also zero and infinity. Thus we hare k' = ir^'" r^''e-'^'+''^xdxdL Since the limits are constants, the order of integration is indifEerent. Let us then first integrate with respect to x. Since xdx = \d-x^ = 2(1^1') ^' (1 + *')'^'> the integral with respect to x is Then, integrating with respect to t. Hence / e~'''dx = V7r. £ BECTIFICATION OF OUBVES. 285 CHAPTER VIII. RECTIFICATION AND QUADRATURE. 177. The Rectification of Curves. In the older geometry to rectify a curve meant to find a straight line equal to it in length. In modern geometry it means to find an algebraic expression for any part of its length. Let us put s for the length of the curve from an arbitrary fixed point C to a vari- able point P- If P' be another position of the variable point, we shall then have no. so. As = PP'. If PP' becomes infinitesimal, it has already been shown (§ 79) that we have, in rectangular co-ordinates. ds = Vdd' + dy^ = /l + i^£ldx = 1/1+ i^)dy, (1) and, in polar co-ordinates, If both co-ordinates, x and y, are expressed in terms of a third variable u, we have *■ = &»)■+ (I* )^ The length of any part of the curve is then expressed by 286 THE INTEGRAL CALCULUS. the integral of any of these expressions taken between the proper limits. Thus we have or •=/{S)+(f)T- (3) In order to effect the integration it is necessary that the second members of (3) shall be so reduced as to contain no other variable than that whose differential is written; that is, we must have ds=f{x)dx; f{y)dy; f{e)dd; or f{u)du. Then we take for the limits of integration the values of X, y, 6 or u, which correspond to the ends of the curve. 178. Rectification of the Parabola. From the equation of the parabola y' = 2px we derive ydy = pdx. We shall have the simplest integration by taking y as the independent variable. We then have rfs = 1 1 +(|)' I 'dy; pds = \p' + y']'dy. The formula ((?) of § 145 gives /(/ + y')'dy = iy{p' + y')* + Wf^l ^^ {a) (y+y')' ,«\*" The method of § 132 gives dy fr-rir^i = A - log {{p' + ff - y) "^ {p +y) = A - log ^ + log (( p' + y')* + y). BECTIFICATION OF CUBVES. 287 Thus, putting h' = ip {h — log^), the indefinite integral of (a) is s = h' + i|(/ + f)' + ^p log ((y + ff + y). The arbitrary constant h' must be so taken that s shall vanish at the initial point of the parabolic arc. If we take the vertex as this point, we must have s — Qloxy = 0. Then h' - -ip log p. We therefore have, for the length of a parabolic arc from the vertex to the point whose ordinate is y, s = \lif + ff + \plo,i^+f±JL. (4) 179. Rectification of the Ellipse. The formulae for rec- tifying the ellipse take the simplest form when we express the co-ordinates in terms of the eccentric angle u; then (Analyt. Geom. ) a; = a cos m; y = b sin u. We then have dx = — a sin udu; dy = h cos udu. Then if e is the eccentricity, so that a'e' = a' — V, ds = (a' sin' u -\-b' cos' uydzi = «(1 — e' cos' u)*du; s = a / {1 — e' cos' «)* du. This expression can be reduced to an elliptic integral: a kind of function which belongs to a more advanced stage of the calculus than that on which we are now engaged. It may, however, be approximately integrated by develop- ment in series. We have, by the binomial theorem, (1 — e' cos' m)* = 1 — s-e' cos' u — ^r— ; e* cos* u — a.A.a ^ COS U — etc. 288 THE INTEQBAL CALCULUS. The terms in the second member may be separately in- tegrated by the formulae (6), § 149, by putting m = and n — 'H, i, 6, etc. We thus find 2 / 008° udu — sin u cos u-\-u; 4 / cos* udu = sin m(cos'' if + f cos u) -\- fw; etc. etc. etc. Since at one end of the major axis we have m = and at the other end u = re, we find the length of one half of the ellipse by integrating between the limits and n. Since sin u vanishes at both limits, we have j cos' udu = x-7r; 1"3 cos' udu = :r-^;r; 3-4 1-3-5 We thus find by substitution that the semi-circumference of the ellipse may be developed in powers of the eccentricity with the result A 1 , 3 . y-b . \ s = a7r\l - -,e - ^^,e - ^^r:^e -. . .j. 180. T/ie Cycloid. The co-ordinates x and y of the cy- cloid are expressed in terms of the angle u through which tlie geiierating circle has moved by the equations (§80) X =? «(«< — sin m); y = a(l — cos m). Hence ds' = dx* + dy' = «'{(! - cos uf + sin' u\du' — 3a' (1 — cos u)du,^ = 4a' sin' ^u.du''. By extracting the root and integrating, s = h — 4,a cos iu. BBCTIFICATION OF 0URYB8. 289 If we measure the arc generated from the point where it meets the axis of abscissas, that is, where w = 0, we must have s = f or ?{ = 0. This gives and s = 4a(l — cos ^m) = 8a sin" ^. This gives, for the entire length of the arc generated by one revolution of the generating circle, s = 8a; that is, four times the diameter of the generating circle. 181. The Archimedean Spiral. From the polar equation of this spiral (§ 83) we find dr = add. Hence ds = a(l + d'fdd. Then the indefinite integral is (§ 147, Ex. 1) s = 1 1 «(i + n' + log o{d +(1 + e')f [ . If we measure from the origin we must determine the value of by the condition that when 6 = 0, then s = 0. This gives log (7=0; . ■ . C = 1. If instead of 6 we express the length in terms of r, the radius vector of the terminal point of the arc, we shall have s = ^-{a+r) +^logi -> . 183. The Logarithmic Spiral. The equation of this spiral (§ 83) gives ^a = ale = Ir. da Hence ds = {l-\- Vfrdd. To integrate this differential with respect to 6 we should first substitute for r its value in terms of 6. But it wUl be 19 290 THE INTEGRAL CALCULUS. better to adopt tlie inverse course, and express d6 in terms of dr. We thus have (1 + I'Y ds = = dr', (1 + l\i whence s = ^ — —r + *»> s„ being the value of s for the pole. If we put y for the constant angle between the radius vector and the tangent, then (§§ 90-92) ;=cot y, and we have s = r sec 7 + *o' Between any two points of the curve whose radii-vectors are r„ and r, we have s = (r, — r„) sec y. Hence the length of an arc of the logarithmic spiral is pro- portional to the difference iefween the radii-vectors of the ex- tremities of the arc. EXERCISE. 1. Show that the differential of the arc of the lemniscate is add Vl - 2 sin' e (This expression can be integrated only by elliptic func- tions.) 183. The Quadrature of Plane Figures. In geometrical construction, to square a figure means to find a square equal to it in area. The operation of squaring is called quadrature. In analysis, quadrature means the formation of an algebraic expression for the area of a surface. In order to determine an area algebraically, the equation of the curve which bounds it must be given. Moreover, in order that the area may be completely determined by the bounding line, the latter must be a closed curve. Then whatever the form of this curve, every straight line QUADRATURE OV PLANE MQURES. 291 s / T / / o v —a Xo Xi Fio. 51. must intersect it an even number of times. The simplest case is that in which a line paral- lel to the axis of Y cuts the bound- ary in two points. Then for every value of x the equation of the curve will give two values of y corresponding to ordinates termi- nating at P and Q. Let these values be y„ and y^. Then, the infinitesimal area in- cluded between two ordinates infinitely near each other will be The area given by integrating this expression will be in which the limits of integration are the extreme values of z corresponding to the points X„ and X„ outside of which the ordinate ceases to cut the curve. The same principle may be applied by taking (a;, — x^dy as the element of the area. We then have o- = r\x, - x,)dy. If the curve is referred to polar co-ordinates, let S and T be two neighboring points of the curve, and let us put r=OS; r'=OT; /I e = angle SOT. If we draw a chord from S to T, the area included between this chord and the curve will be of the third order (§ 78). The area of the triangle formed by this chord Fig. ! 292 THE INTEGRAL CALCULUS. and the radii vectors will be ^rr' sin Ad. Now let /id be- come infinitesimal. OS will then approach r as its limit; the ratio of sin /J^ to Ad itself will approach unity, and the area of the triangle will approach that of the sector. Thus we shall have, for the differential of area, da = ^"dd. If the pole is within the area enclosed by the curve, the total area will be found by integrating this expression be- tween the limits 0° and 360°. Thus we have, for the total area. 'dd. 184. The Parabola. As the parabola is not itself a closed curve, it bounds no area. But we may find the area of any segment cut off by a double ordinate MH. The equation of the curve gives, for the two values of y, y, = + V2px; Hence y« da = V^. x*dx. The indefinite integral is Fio. S3 a = iV2^+ C. For the area from the vertex to JO^we put a;, = OX, and take the integral between the limits and a;,. Calling this area a^, we have 0", = ^ 1^3^.2;, = ^x^y^ = fa;, X2y,. Because 2y, = il/iV, it follows that the area^^J/7V= 2a;,y,. Hence: Theokeit. The area of a parabolic segment is two thirds that of its circumscribed rectangle. QUADRATURE OF PLANE FIGURES. 293 185. The Circle and the Ellipse. Eef erring the circle of radius a to the centre as the origin, the values of y will be y=±{a'- cc')K Hence fiy,-y„)dx = 2f(a'-xydx X = x(a' - x')i + a' sin <-«- + h. ^ ' a This expression, taken between appropriate limits, will give the area of any portion of the circle contained between two ordinates. Taking the integral between the limits — a and + « gives, for the area of the circle, o- = a' sin<-« (+ 1) - a' sin'"" (- 1) = na\ The Ellipse. From the equation of the ellipse referred to its centre and axes, namely, El + ^-l we find V = ± - Va' — a;'. •^ a The entire area will be / + o 5 /1 + a , • a Cio —a The last integration is performed exactly as in the case of the circle. 186. The Hyperbola. Since the hyperbola is not a closed curve, it does not by itself enclose any area. But we may consider any area enclosed by an hyperbola and straight lines. Let us first consider the area APM contained between the curve, the ordinate MP, and the segment AM ot the major 294 THE INTEGRAL CALCULUS. axis. The equation of the hyperbola referred to its centre and axes gives, for the value of y in terms of x. ^ a If we put a;, for the value of the abscissa OM, then, since OA — a, the area AMF will be equal to the integral - f^\x' - a')*dx; Fio. 64. and for the definite integral between the limits a and x, Area APM- 1 te 3 a (z' - a") ah "2 Now, ^xy is the area of the triangle 0PM; we therefore conclude that the second term of the expression is the area included between OA, OP and the hyperbolic arc AP. Much simpler is the area included between the curve, one asymptote, and two parallels to the other asymptote. The equation of the hyperbola re- ferred to its asymptotes as axes of co-ordinates (which axes are oblique unless the hyperbola is equilateral) may be reduced to the form fio. 55. xy = ab 2 sin o^ QUADRATURE OF PLANE FIGURES. 295 a being the angle between the axes. We readily see that the differential of the area is ydx X sin a instead of ydx simply. Hence for the area we have jy sin adz = j ^-dx = -^ log ex. If we take the area between the limits OM=x„ and OM = a;,, the result will be /^i ab -, db ., X. X Tx^^^^ii''^^: We note that this area becomes infinite when a;„ becomes zero or when a;, becomes infinite, showing that the entire area is infinite. 187. The Lemniscate. The equation of this curve in polar co-ordinates is (§ 81) r° = a' cos 2ft It will be noted that r becomes imaginary when 6 is con- tained between 45° and 135°, or between 235° and 315°. The integral expression for the area is ^fr'dd = ia'fcos 2edd = Ja' sin 2(9. To find the area of the right-hand loop of the curve we must take this integral between the limits = —i5° and 6 = -h 45°, for which sin 2(9 = - 1 and -f 1. Hence Half area = ^d'; Total area = a'. Hence the area of each loop of the lemniscate is half the square on the semi-axis. 188. The Cycloid. By differentiating the expression for the abscissa of a point of the cycloid we have dx = a{l — cos u)du. Hence 296 THE INTEGRAL CALCULUS. Cydx =a' /"(l— cos uydu=a'' y (|— 3 cos m + -J cos %u)du. The indefinite integral is |«{ — 3 sin M + i sin 2v. To find the whole area we take the definite integral between the limits and 2;r. Thus we find Area of cycloid = 3;ra% or three times the area of the generating circle. EXERCISES. I. Show that the theorem of § 184 is true only of the pa- rabola. To do this we must find what the equation of a curve must be in order that the theorem may be true. The theorem is J ydx- Differentiating both members, we have y^ = ixdy + • -0 — ^ ' ' y x' Then, integrating both members, log y'' = log ex ; .• . ^ = ex, c being an arbitrary constant. This is the equation of a parabola whose parameter is ic. 2. Show that the equation of a curve the ratio of whose area to that of the circumscribed rectangle ia m : n must be of the form OUBATUBE OF VOLUMES. 397 CHAPTER IX. THE CUBATURE OF VOLUMES. 189. General Fortnulce for Gubature. In the ancient Geometry to cube a solid meant to find the edge of a cube whose volume should be equal to that of the solid. In Ana- lytic Geometry it means to find an expression for the volume of a solid. Let us have a solid the bounding surface of which is de- fined by an equation between rectangular co-ordinates. Let the solid be cut by a plane PL parallel to the plane of TZ, and let u be the area of the plane section thus formed. If we now cut the solid by a second plane, parallel to PL and infinitely near it, that portion of the solid contained between the planes will be a slice of area u and thickness dx, dx being the infinitesimal distance between the planes. If, then, we put v for the volume of that part of the solid contained between any two planes parallel to YZ, we have Tie. so. and dv = udx, w = / udx, (1) a:„ and a;, being the distances of the cutting planes from the prigin 0, 298 THE INTEGRAL CALCULUS. If we take for a;„ and a;, the extreme values of x for any part of the solid, the aboTe expression will give the total vol- ume of the solid. In order to integrate (1), we must express m as a function of X. That is, we must find a general expression in terms of X for the area of any section of the solid by a plane parallel to that of XT. This is to be done by the equation of the bounding surface of the solid. Of course we may form the infinitesimal slices by planes perpendicular to the axis of Y ot of Z as well as of X, 190. The Sphere. The equation of a sphere referred to its centre as the origin is _ [Zj yy a;' + y' + a' = a'. If we cut the sphere by a plane PMQ parallel to the plane of YZ, and having the abscissa OM = X, the equation of the circle of intersection will be y' + z' = a" — a?; that is, the radius MP of the circle will be Va^ — x', and its area will be 7t{a' — a;'). Hence the differential of the vol- ume of the sphere will be dv = n{a^ — x')dx, and the indefinite integral will be V = 7r{a'x — ix') +0. The extreme limits of x for the sphere are x„= — a and a;, = + «. Taking the integral between these limits, we have Volume of sphere = ^yra', Fio. 57. OVBATURE Of VOLUMES. 200 Let the pyramid be placed 191. Volume of Pyramid. witli its vertex at the ori- gin, and its base parallel to the plane of XT. Let ub also put h = OZ its alti« tude; a, the area of its base. Let it be cut by a plane EFGH parallel to its base. It is shown in Geometry that the section EFGH is similar to the base, and that the ratio of any two homologous sides, as EFaudAS, is the same as the ratio OL : OZ. Because the areas of polygons are proportional to the squares of their homologous sides, .-.Area EFGH : Area ABGD = OL' : OZ'. Putting Area ABGD — a, OL = z and OZ = h, Area. EFGH ^^. Fig. 58. The volume of the pyramid is therefore That is, one third the altitude into the base. The same formulae apply to the cone. 193. TJie Ellipsoid. The equation of the ellipsoid re- ferred to its centre and axes is a, h and c being the principal semi-axes. If we cut the ellipsoid by the plane whose equation is X = x', the equation of the section will be b' '^ c' 1- 300 TBB INTEQUAL CALOULXfa. This is the equation of an ellipse whose semi-axes are ■Vc?^' and V^ Hence its area is ar5c(a' - x") Fio. eg. Then, by integration between the limits —a and +a, we find Volume of ellipsoid = ^nabc. From the known expression for the area of an ellipse (nab) it is readily found that the volume of an elliptic cylinder cir- cumscribing any ellipsoid is "iiTtabc. Hence we conclude: The volume of an ellipsoid is two thirds that of any rigJit elliptic cylinder circumscribed about it. 193. Volume of any Solid of Revolution. In erder that a solid of revolution may have a well-defined volume it must be generated by the revo- lution of. a curve or un- broken series of straight or curve lines terminating at two points, Q and R, of the axis of revolution. As an element of the volume we take two planes infinitely near each other and perpendicular to the axis of revolution. Every such plane cuts the solid in a circle. If we place the origin at 0, take the axis of revolution as that of X, and let OM = a; be the abscissa of any point P of the curve, and MP = y its ordinate, then the section of the solid through M will be a circle of ra- dius y, whose area will therefore be Try'. Hence the volume contained between two planes at distance dx,wii.l be Tty'dx, and the volume between two sections whose abscissas are x„ and a;, will be V = I mfdx. (1) OUBATUBE OW VOLUMES. 301 If the two co-ordinates are expressed in terms of a third variable u by the equations we have X = 0(m), y = ip{u), dx = 0'(m)c?m. Putting M, and m, for the values of u corresponding to a;„ and »„ the expression (1) for the volume will become V=7i r\ip{u)Y(t>'{u)du. (2) The equations (1) and (3) give the volume AA'B'B gen- erated by the revolution of any arc ^.B of the given curve, and of the ordinates MA and NB of the extremities of the arc. The limits of in- tegration for X are OM = x„ and OH = a;,. To find the entire volume generated we must extend these limits to the points (if any) at which the curve intersects the axis of revolution. 194. TJie Paraboloid of Revolution. The equation of the parabola being y' = 2px, we readily find from (1) a result leading to the following theorem, which the student should prove for himself: Theorem. The volume of a para- boloid of revolution is one half that of the circumscribed cylinder. 195. The Volume Generated by the Revolution of a Cycloid around its Base. From the equations of the cycloid in terms of Fio. 61. 302 Tm: INTEGRAL CALCULUS. the angle through which the generating circle has moved, we find the element of the volume to be <?F= 7ta'{i — cos uydu. Hence V = no? / (1 — 3 cos M + 3 cos' u — cos' u)du. By the method of §§ 149, 150, with simple reductions, we find / cos" udu = |m + J sin %u; I cos' udu = / (1 — sin' u)d. sm m = sin m — ^ sin' u = I sin M + -^ sin 3m. We thus find, for the indefinite integral, V = ;ra'(f M — J^- sin m + 1 sin 2m — -^ sin 3m). The total volume formed by the revolution of one arc of the cycloid is found by taking the integral between the limits M = and M = 27r. The volume thus becomes V=57t'a', from which follows the theorem: The volume generated hy the revolution of a cycloid around its base is five eighths that of the circumscribed cylinder. 196. The Hyperboloid of Revolution of Two Nappes. This figure is formed by the revolution of an hyperbola about its transverse axis. The general expression for the volume is found to be F=|^;(a;'-3a'x + A), h being the arbitrary constant of integration. If we consider that part of the infinite solid cut ofE by a plane perpendicular to the transverse axis, we must determine h by the condition CUBATUBE OF VOLUMES. 303 that V shall vanish when x = a, because then the plane will be a tangent at the vertex of the hyperboloid, and the volume wiU become zero. This condition gives n ■= 3a' -a' = ^a\ Thus we have V - ^[(a:' - U'x + 3«') = g(a; - a)\x + 2a). (1) By the same revolution whereby the hyperbola describes an hyperboloid of revolution the asymptotes will describe a cone. Let us compare the volume just found for the hyperboloid with that of the asymptotic cone, cut off by the same plane which cuts off the hyperboloid. The equation of the generat- ing asymptote being ay = hx, we find for the volume of the cone The difference between (1) and (2) will be the volume of the cup-shaped solid formed by cutting the hyperboloid out of the cone. Calling this volume V", we find V" = 7th\x - f«). (3) This is equal to the volume of a circular cylinder of which the diameter is the conjugate axis of the hyperbola, and the altitude x — fa. This result is intimately associated with the following theorem, the proof of which is quite easy: If a plane perpendicular to the axis of revolution cut an hyperbola of two nappes and its asymptotic cone, the area of the plane contained between the circular sections is constant and equal to the area of the circle whose diameter is the con- jugate axis. 804 THE INTEGRAL CALCULUS. 4 p f o M X Fia. 62. 197. Ring-shaped Solids of Revolution. If any com- pletely bounded plane figure APQB revolve around an axis OX lying in its own plane, but wholly outside of it, it will describe a ring-shaped solid. To investigate such a solid, let the ordinate MP cut the figure in the points Q and P, and let us put y, = MQ; y,^MP. The points P and Q will describe two circles which will contain between them the sectional area Taking two ordinates at the infinitesimal distance dx, the corresponding infinitesimal element of volume will be dV=n{y:-y,')dx. (1) The integral V=7t r\y,' - y^)dx = n ["'{y^ + y^) (y, - y^^dx will express the volume of that part of the solid contained be- tween the two planes whose respective abscissas are a;„ and a;,. By taking for x„ and x^ the abscissas of the extreme points A and B, V wUl express the total volume of the solid. 198. Application to the Circular Ring. Let the figure .45 be a circle of radius c, whose centre is at the distance b from the axis of revolution. Let us also put a = the abscissa of the centre. We then have y^ y. + y. y, - y. Vc'- (x - a)'; h + VT'- 2b; {x - a)'; 2Vc'-(x- a)'; CUBATUnE Of VOLUMMB. 305 F= 4;r5 /""[c' - {x - ayfdx. tyXft The limits of integration for the whole volume are a;„ = a — c and x^ — a-\-c. If we put z = x — a, the total volume will become V=4.7iljJ''^\c'-z')\ dz. By substituting the known value of the definite integral, we have The area of the generating circle is 7tc^, and the circumfer- ence of the circle described by its centre is SttS. The product of these two quantities is 'HiTt^hc'. Hence: The volume of a circular ring is equal to the product of the area of its cross-section into the circumference of its central circle. EXAMPLES AND EXERCISES. 1. Compare the cycloid with the semi-ellipse having the same axes as the cycloid, and show the following relations be- tween them: a. The maximum radius of curvature of the ellipse (at the point B) is greater than that of the cycloid in the ratio Ti" : 2i, ox b : 4, nearly. 13. The area of the semi-ellipse is greater than that of the cycloid in the ratio tt : 3. y. The volume of the ellipsoid of revolution around the axis OX is greater than that generated by the revolution of the cycloid in the ratio 16 : 15. 30 306 TEE INTEGRAL CALCULUS. n K Fig. 03. 199. Quadrature of Surfaces of Revolution. Let us put q^ .^s = a small arc PQ oi a, curve re- volving round an axis OX; y = the distance of F from the ' axis OX; y' = the distance of Q from the p'v axis OX. Considering Js as a straight line, the surface generated by it will be the curved surface of the frustum of a cone. If we put /1(T = the area of this curved surface, we have, by Geometry, A(T = 7t{y + y')^s. Now let As become infinitesimal. Then y' will approach y as its limit, and we shall have, for the differential of the sur- face. d(T = 'Hityds ■■ 2^2/[l + This expression, when integrated between the limits a;„ and a;,, will give the area of that portion of the surface for which the co-ordinates x are contained between «;„ and a;,. The modifications and transformations of this formula so as to apply it to cases when another axis than that of Y is the axis of revolution, or when the equation of the curve is not in the form y = <p{x), can be made by the student himself. 300. Examples of Surfaces of Revolution. The process of applying the general formula for da- to special cases is so like that already followed in quadrature and cubature that the briefest indications will suffice to guide the student. Surface of the Sphere. Supposing the equation of the gen- erating circle to be written in the form x'^y' = a\ SUBFA0E8 OF REVOLUTION. 307 we shall find the differential of the surface to be d<T = %7tadx. Prom this we may easily prove the following : Theorem I, If a sphere be cut by any number of parallel and equidistant planes, the curved surfaces of the spherical zones contained between the planes will all be equal to each other. Theoeem II. Tlie total surface of a sphere is equal to the product of its diameter and circumference. Surf ace generated by the Revolution of a Cycloid. We shall find the differential of the surface to be d(y = ^Tta' sin' ^du. By a formula found in Trigonometry, we have 8 sin' V = 6 sin w — 3 sin 3w. Hence, putting v = ^m, d<r — ^ltco' (3 sin v — sin dv)dv. The whole surface is obtained by integrating between the limits M = and ?< = 27r; that is, v = and v — it. We thus find, for the total surface, a = s^Tta'. Hence the theorem: The total surface generated by the revolution of a cycloid about its base is four thirds the surface of the greatest in' scribed sphere. The Paraboloid of Revolution. Taking the integral be- tween the limits zero and x, we have for the curved surface THE END, Cornell Uttfemitg Jitotg THE ©I FT OF 3w|.J?,,C., Ca^I«/. 4.VJI1I : tf/a/tf y ^ ^ 'i^t jK'ff/' t 'y /-/' if •